pubsub/rabbitpubsub: add query string set the qos prefetch count

[peczenyj]

Fixes #3430

Since we usually want to set the prefetch count, I simplify the rabbit url to support a single query string

we can use rabbit://myqueue (continues backward compatible) or the new one rabbit://myqueue?prefetch_count=<prefetch-count-value>

I tested locally using a rabbitmq docker image and it seems to work properly.

[codecov]

Codecov Report

Attention: Patch coverage is 72.41379% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 73.26%. Comparing base (d8b9c94) to head (ca2c5a2).

Files	Patch %	Lines
pubsub/rabbitpubsub/rabbit.go	77.77%	4 Missing and 2 partials ⚠️
pubsub/rabbitpubsub/amqp.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3431      +/-   ##
==========================================
+ Coverage   73.21%   73.26%   +0.04%     
==========================================
  Files         113      113              
  Lines       14873    14897      +24     
==========================================
+ Hits        10890    10914      +24     
+ Misses       3211     3207       -4     
- Partials      772      776       +4     

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