feat: websocket: support no response.

[tttoad]

Resolves #346

https://opentelemetry.io/docs/specs/opamp/#websocket-message-exchange
The specification does not require that we must respond to client messages in WebSocket, and we should support not responding to messages. This will reduce sending empty responses.

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 78.45%. Comparing base (e718373) to head (0d741ce).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
server/serverimpl.go	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #355      +/-   ##
==========================================
- Coverage   78.50%   78.45%   -0.05%     
==========================================
  Files          25       25              
  Lines        2405     2423      +18     
==========================================
+ Hits         1888     1901      +13     
- Misses        410      414       +4     
- Partials      107      108       +1     

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[tigrannajaryan]

We also need to handle the case OnMessage() returning nil in handlePlainHTTPRequest. However, there we MUST send a response (because plain HTTP), so if nil, we must substitute it by empty message. And add a test for plain http too - ideally use one test iterating over both transports.

[tigrannajaryan]

Please also update doc-comment for OnMessage. It currently says "Must return a ServerToAgent message...".

[tttoad]

Done.