add feature online plugins filter flag

[AntoineJac]

Add a new flag to only validate plugins online

[mheap]

Deck is adding lots of these "only X" or "skip Y" flags. Can we make this more generic, so that they can pass something like:

--validate-only plugins,consumers?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 60 lines in your changes missing coverage. Please review.

Project coverage is 27.59%. Comparing base (4da4246) to head (852694d).

Files with missing lines	Patch %	Lines
validate/validate.go	0.00%	37 Missing ⚠️
cmd/gateway_validate.go	0.00%	23 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1458      +/-   ##
==========================================
+ Coverage   27.55%   27.59%   +0.04%     
==========================================
  Files          61       61              
  Lines        6294     6283      -11     
==========================================
  Hits         1734     1734              
+ Misses       4432     4421      -11     
  Partials      128      128              

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[AntoineJac]

@mheap sure I was actually thinking about something similar first. Let me refactor this PR

For info it really does have a nice impact on performance, here a test with a small file:

Performance are almost 40% to 50% better

[AntoineJac]

@mheap can you confirm it looks ok or do you want me to edit the flag name? I will then ask for review for this PR.
I have also slightly refactor to simplify the code and use a EntityMap global var instead of running test by test

Thanks

FlagName: --online-entities-list

Entities should be part of the EntitiesMap: RBACEndpointPermissions, RBACRoles, Plugins, Services, ACLGroups, BasicAuths, CACertificates, Certificates, Consumers, Documents, HMACAuths, JWTAuths, KeyAuths, Oauth2Creds, Routes, SNIs, Targets, Upstreams, FilterChains, Vaults.

[AntoineJac]

Any update on this? Thanks

[mheap]

@Prashansa-K Please review

Done