Idempotency key for broker publish

[FZambia]

This should allow dropping duplicate publications on Centrifuge/Centrifugo level based on user-provided idempotency key

node.Publish("channel", []byte(`{}`), WithIdempotencyKey("my-key"))

Idempotency keys work per channel, default idempotency key cache time window is 5 minutes

[codecov]

Codecov Report

Attention: 45 lines in your changes are missing coverage. Please review.

Comparison is base (ef7d6b2) 83.87% compared to head (26417c0) 83.71%.

Files	Patch %	Lines
broker_redis.go	59.78%	29 Missing and 8 partials ⚠️
broker_memory.go	87.69%	6 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #336      +/-   ##
==========================================
- Coverage   83.87%   83.71%   -0.16%     
==========================================
  Files          38       38              
  Lines        7472     7603     +131     
==========================================
+ Hits         6267     6365      +98     
- Misses        893      917      +24     
- Partials      312      321       +9     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.