Add deprecation warnings

Motivation:

v2 has moved to a new repo, grpc-swift-2. That's not all that
discoverable.

Modifications:

- Deprecate commonly used high-level types with a link to a forums post
  explaining the move.

Result:

Users are notified about the move

Author: glbrntt

Sources/GRPCCore/Call/Server/RegistrableRPCService.swift

@@ -23,6 +23,7 @@
 /// ``RPCRouter/registerHandler(forMethod:deserializer:serializer:handler:)`` for each method
 /// you want to register with the router.
 @available(gRPCSwift 2.0, *)
+@available(*, deprecated, message: "See https://forums.swift.org/t/80177")
 public protocol RegistrableRPCService: Sendable {
   /// Registers methods to server with the provided ``RPCRouter``.
   ///

Sources/GRPCCore/GRPCClient.swift

@@ -55,6 +55,7 @@ private import Synchronization
 /// additional resources that need their lifecycles managed you should consider using [Swift Service
 /// Lifecycle](https://github.com/swift-server/swift-service-lifecycle).
 @available(gRPCSwift 2.0, *)
+@available(*, deprecated, message: "See https://forums.swift.org/t/80177")
 public final class GRPCClient<Transport: ClientTransport>: Sendable {
   /// The transport which provides a bidirectional communication channel with the server.
   private let transport: Transport
@@ -399,6 +400,7 @@ public final class GRPCClient<Transport: ClientTransport>: Sendable {
 ///   - handleClient: A closure which is called with the client. When the closure returns, the
 ///       client is shutdown gracefully.
 @available(gRPCSwift 2.0, *)
+@available(*, deprecated, message: "See https://forums.swift.org/t/80177")
 public func withGRPCClient<Transport: ClientTransport, Result: Sendable>(
   transport: Transport,
   interceptors: [any ClientInterceptor] = [],
@@ -428,6 +430,7 @@ public func withGRPCClient<Transport: ClientTransport, Result: Sendable>(
 ///       client is shutdown gracefully.
 /// - Returns: The result of the `handleClient` closure.
 @available(gRPCSwift 2.0, *)
+@available(*, deprecated, message: "See https://forums.swift.org/t/80177")
 public func withGRPCClient<Transport: ClientTransport, Result: Sendable>(
   transport: Transport,
   interceptorPipeline: [ConditionalInterceptor<any ClientInterceptor>],

Sources/GRPCCore/GRPCServer.swift

@@ -76,6 +76,7 @@ private import Synchronization
 /// Lifecycle](https://github.com/swift-server/swift-service-lifecycle) and the
 /// `GRPCServiceLifecycle` module provided by [gRPC Swift Extras](https://github.com/grpc/grpc-swift-extras).
 @available(gRPCSwift 2.0, *)
+@available(*, deprecated, message: "See https://forums.swift.org/t/80177")
 public final class GRPCServer<Transport: ServerTransport>: Sendable {
   typealias Stream = RPCStream<Transport.Inbound, Transport.Outbound>
 
@@ -259,6 +260,7 @@ public final class GRPCServer<Transport: ServerTransport>: Sendable {
 ///       server is shutdown gracefully.
 /// - Returns: The result of the `handleServer` closure.
 @available(gRPCSwift 2.0, *)
+@available(*, deprecated, message: "See https://forums.swift.org/t/80177")
 public func withGRPCServer<Transport: ServerTransport, Result: Sendable>(
   transport: Transport,
   services: [any RegistrableRPCService],
@@ -291,6 +293,7 @@ public func withGRPCServer<Transport: ServerTransport, Result: Sendable>(
 ///       server is shutdown gracefully.
 /// - Returns: The result of the `handleServer` closure.
 @available(gRPCSwift 2.0, *)
+@available(*, deprecated, message: "See https://forums.swift.org/t/80177")
 public func withGRPCServer<Transport: ServerTransport, Result: Sendable>(
   transport: Transport,
   services: [any RegistrableRPCService],

Sources/GRPCCore/Metadata.swift

@@ -80,6 +80,7 @@
 /// the "-bin" suffix may have string values (rather than binary). These are deserialized automatically when
 /// using ``subscript(binaryValues:)``.
 @available(gRPCSwift 2.0, *)
+@available(*, deprecated, message: "See https://forums.swift.org/t/80177")
 public struct Metadata: Sendable, Hashable {
 
   /// A metadata value. It can either be a simple string, or binary data.

Sources/GRPCCore/Transport/ClientTransport.swift

@@ -26,6 +26,7 @@
 /// transport built on top of SwiftNIO in the https://github.com/grpc/grpc-swift-nio-transport
 /// package.
 @available(gRPCSwift 2.0, *)
+@available(*, deprecated, message: "See https://forums.swift.org/t/80177")
 public protocol ClientTransport<Bytes>: Sendable {
   /// The bag-of-bytes type used by the transport.
   associatedtype Bytes: GRPCContiguousBytes & Sendable

Sources/GRPCCore/Transport/ServerTransport.swift

@@ -23,6 +23,7 @@
 /// transport built on top of SwiftNIO in the https://github.com/grpc/grpc-swift-nio-transport
 /// package.
 @available(gRPCSwift 2.0, *)
+@available(*, deprecated, message: "See https://forums.swift.org/t/80177")
 public protocol ServerTransport<Bytes>: Sendable {
   /// The bag-of-bytes type used by the transport.
   associatedtype Bytes: GRPCContiguousBytes & Sendable

Sources/GRPCInProcessTransport/InProcessTransport.swift

@@ -17,6 +17,7 @@
 public import GRPCCore
 
 @available(gRPCSwift 2.0, *)
+@available(*, deprecated, message: "See https://forums.swift.org/t/80177")
 public struct InProcessTransport: Sendable {
   public let server: Self.Server
   public let client: Self.Client