Fixes #12266 - InvocationType improvements and cleanups.

[sbordet]

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• Removed usages of AbstractConnection.getInvocationType().
• Changed HTTP server-side Connection implementations to use AbstractConnection.fillInterested(Callback) with a callback that specifies the InvocationType, derived from the Server, which derives it from the Handler chain.
• Changed client-side receivers to use AbstractConnection.fillInterested(Callback) with a callback that specifies the InvocationType, derived from the HttpClientTransport.
• Introduced HttpClientTransport.getInvocationType(Connection), so that client applications can specify whether they block or not.
• Made sure client-side HTTP/2 and HTTP/3 return tasks with the proper InvocationType to be run by the ExecutionStrategy when calling application code.
• HTTP3StreamConnection now uses an EITHER fillable callback to possibly process streams in parallel.
• QuicStreamEndPoint now uses a task to invoke FillInterest.fillable(), rather than invoking it directly, therefore honoring the InvocationType of callback set by the Connection associated with the QuicStreamEndPoint.