gRPC: Use trait to access node Database (GrpcReader / GrpcWriter)

[muXxer]

We should use a trait to access the storage layer, that makes it easier to implement gRPC in simulacrum for example. See "RestReader" etc,.

[bingyanglin]

Now we have prepared the initial implementation of using the trait to access the storage layer, and also implemented the gRPC in simulacrum, with a test case.
The implementation is at https://github.com/iotaledger/iota/tree/feat/grpc-trait
This PR should be merged after the two following PRs:

• feat(iota-core): gRPC PoC to replace the REST-API for the checkpoint fetching of indexer #7511
• feat(gRPC): Add subscription to events including filtering #7746

To run the examples of simulacrum with gRPC:

# Basic gRPC subscription with checkpoints
cargo run --example basic_grpc_subscription

# Event filtering with simulacrum
cargo run --example grpc_event_client

# Complete workflow demonstration
cargo run --example complete_grpc_workflow