Implement RingCentral notifier

[uderik]

This adds the ability to send flagger notifications to a RingCentral chat.
Tested in our infrastructure.
RingCentral Incoming Webhooks

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 91.52542% with 5 lines in your changes missing coverage. Please review.

Project coverage is 39.56%. Comparing base (2c4b7a6) to head (86de823).
Report is 24 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/notifier/ringcentral.go	94.73%	2 Missing and 1 partial ⚠️
pkg/notifier/factory.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1773      +/-   ##
==========================================
+ Coverage   39.42%   39.56%   +0.13%     
==========================================
  Files         284      285       +1     
  Lines       22422    22481      +59     
==========================================
+ Hits         8840     8894      +54     
- Misses      12632    12636       +4     
- Partials      950      951       +1     

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[uderik]

Hi @stefanprodan , what needs to be done for this PR to be accepted?