chore: adding warning for alpha flags

[JaydipGabani]

What this PR does / why we need it:

Which issue(s) this PR fixes (optional, using fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when the PR gets merged):
Fixes #3485

Special notes for your reviewer:

[JaydipGabani]

@open-policy-agent/gatekeeper-maintainers There are other non-boolean alpha flags such as audit-chunk-size, which have been in alpha for a very long time, do we want to log warnings for such flags? if so, since these are non boolean flags, how do we determine if they are set by users or are being used by default?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 47.70%. Comparing base (3350319) to head (c87f961).
Report is 230 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/audit/manager.go	0.00%	1 Missing ⚠️
pkg/controller/pubsub/pubsub_config_controller.go	0.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (3350319) and HEAD (c87f961). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (3350319)	HEAD (c87f961)
unittests	2	1

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3764      +/-   ##
==========================================
- Coverage   54.49%   47.70%   -6.80%     
==========================================
  Files         134      236     +102     
  Lines       12329    19929    +7600     
==========================================
+ Hits         6719     9507    +2788     
- Misses       5116     9529    +4413     
- Partials      494      893     +399     

Flag	Coverage Δ
unittests	47.70% <66.66%> (-6.80%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

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

[ritazh]

what about all other alpha flags? https://github.com/search?q=repo%3Aopen-policy-agent%2Fgatekeeper%20%22(alpha)&type=code

[JaydipGabani]

@ritazh Yeah those are non-boolean flags, so I am not sure when to log since non-boolean alpha flags are always in use. And if we add warnings for them now, it might alarm users and mislead them since some of these flags have been in alpha for very long time.

[ritazh]

@ritazh Yeah those are non-boolean flags, so I am not sure when to log since non-boolean alpha flags are always in use. And if we add warnings for them now, it might alarm users and mislead them since some of these flags have been in alpha for very long time.

emit-audit-events and enable-pub-sub are boolean flags and default to false.

[JaydipGabani]

@ritazh Yeah those are non-boolean flags, so I am not sure when to log since non-boolean alpha flags are always in use. And if we add warnings for them now, it might alarm users and mislead them since some of these flags have been in alpha for very long time.

emit-audit-events and enable-pub-sub are boolean flags and default to false.

Warning for enable-pub-sub when set to true is being added in this PR. I will add warning for emit-audit-events as well.

[maxsmythe]

LGTM