Fix sendability issues in the connection pool (#833)

Motivation:

The connection pool holds much of the low level logic in AHC. We should
fix its sendability issues before moving to higher levels.

Modifications:

- Make HTTP1ConnectionDelegate and HTTP2Delegate sendable, this requires
passing IDs rather than connections to their methods
- Make HTTPConnectionRequester sendable and have its methods take
Sendable views of the HTTP1Connection and HTTP2Connection types
- Add sendable views to HTTP1Connection and HTTP2Connection
- Mark HTTP1Connection and HTTP2Connection as not sendable
- Make HTTPRequestExecutor and HTTPExecutableRequest sendable
- Update tests

Result:

Connection pool has stricter sendability requirements

Sources/AsyncHTTPClient/ConnectionPool/HTTPExecutableRequest.swift

@@ -176,7 +176,7 @@ protocol HTTPSchedulableRequest: HTTPExecutableRequest {
 /// A handle to the request executor.
 ///
 /// This protocol is implemented by the `HTTP1ClientChannelHandler`.
-protocol HTTPRequestExecutor {
+protocol HTTPRequestExecutor: Sendable {
     /// Writes a body part into the channel pipeline
     ///
     /// This method may be **called on any thread**. The executor needs to ensure thread safety.
@@ -201,7 +201,7 @@ protocol HTTPRequestExecutor {
     func cancelRequest(_ task: HTTPExecutableRequest)
 }
 
-protocol HTTPExecutableRequest: AnyObject {
+protocol HTTPExecutableRequest: AnyObject, Sendable {
     /// The request's logger
     var logger: Logger { get }