Refactor benchmark layout, remove non-actual examples

[FZambia]

Some changes here:

• move bench client from centrifuge-go to _examples here - should be more visible and simpler to use.
• remove benchmark_gobwas, benchmark_nhooyr - do not have a lot of sense these days when we use Gorilla Websocket fork and no current plans to switch builtin implementation to sth different
• remove custom_ws_gobwas, custom_ws_nhooyr examples - according to benchmarks done I don't think they can provide any benefit over Centrifuge built-in WebSocket implementation, so better to now push Centrifuge users in this direction.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 83.48%. Comparing base (e582c1f) to head (a6042da).

Files with missing lines	Patch %	Lines
client.go	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #482      +/-   ##
==========================================
- Coverage   83.65%   83.48%   -0.17%     
==========================================
  Files          39       39              
  Lines        9041     9047       +6     
==========================================
- Hits         7563     7553      -10     
- Misses       1118     1132      +14     
- Partials      360      362       +2     

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