Refactor HttpHandlers to use fewer arguments and to stream responses

[jschmidt-icinga]

This PR is a work in progress. It fixes #10142 and is a step towards #10409.

This is based on the improvements to @yhabteab's improvements to the JSON encoder in #10414.

Description

• Added a new class HttpRequest that incorporates all of the derived data, like the authenticated user, url and parameters, that the handlers need, so the don't have to be passed as individual arguments, as per the suggestion in Simplify HttpHandler::HandleRequest() signature by grouping arguments into structs #10142.
• Added the new class HttpResponse, supported by a new custom message body type that makes it easy to use the boost::beast::http::serializer to start sending some responses with chunked encoding, before the complete data and content_length information is available.
• These classes derive from the respective boost::beast:message types and are usable in functions that require these types.
• In regard to the HttpHandler interface parameters, this leaves only the WaitGroup, the request and response objects and the yield_context being passed to the handlers.
• To support seamless disconnect in EventsHandler but without adding a solution specific to it (like StartStreaming did previously), I've added a third coroutine to HttpServerConnection that waits for the stream to be shut down by the remote side and then initiates a Disconnect().
• I added unit-tests for HttpServerConnection, HttpRequest and HttpResponse. These add lots of functionality that can be used to extend unit-tests to other components (such as the individual HTTP handlers, JsonRpcConnection, etc.) in the future.

Todo

• The EventsHandler currently still blocks formally disconnecting the connection until the next event is returned by the EventsInbox. Before this PR, that was handled in HttpServerConnection::StartStreaming() by trying to read from the connection in another coroutine and calling HttpServerConnection::Disconnect() when the connection is closed. The handler was still sitting there, but it could simply return when it got the next event while the connection was already disconnected.
• The signalling between the coroutines needs some more work. Possibly this can be done more implicitly. Specifically I want to get rid of HttpResponse::Begin() and HttpResponse::Done().
• Get distros using boost-1.66 (opensuse/leap, rockylinux:8) to build. 1.66 was the first version boost::beast was introduced and there are some pesky differences to the interface of the message body implementation needs to satisfy.
• Look into adding test-cases for Http(Request|Response) and possibly HttpServerConnection (minimal version without handlers)
• Look at more handlers and see if they can benefit from using these new features.
• Some other smaller things (cleaning up commits, improving documentation, etc.)

[Al2Klimov]

In addition to #10491 (review):

[jschmidt-icinga]

I'm addressing the remaining comments and then I'm going to close this draft PR and open an new one for proper review, mostly so we don't have to scroll past 100 force pushes and resolved conversions.