Support custom entities in Railgun

[rainest]

What this PR does / why we need it:
Add the kong-custom-entities-secret flag to Railgun, load the specified Secret, and pass it to sendconfig.PerformUpdate().

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #1306

Special notes for your reviewer:
Testing: good question! For https://docs.konghq.com/kubernetes-ingress-controller/1.2.x/guides/configuring-custom-entities/ you need a plugin that actually defines custom entities. The core plugin suite doesn't really have any of these--oauth2 does but it's not available in DB-less mode. We're kinda stuck for custom entity integration tests unless we want to build a custom Kong image that includes such a plugin.

[codecov]

Codecov Report

Merging #1339 (7751650) into next (9075a8d) will increase coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             next    #1339      +/-   ##
==========================================
+ Coverage   55.60%   55.74%   +0.13%     
==========================================
  Files          38       38              
  Lines        3566     3577      +11     
==========================================
+ Hits         1983     1994      +11     
  Misses       1434     1434              
  Partials      149      149              

Impacted Files	Coverage Δ
...n/internal/proxy/clientgo_cached_proxy_resolver.go	63.72% <100.00%> (+4.38%)	⬆️

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[shaneutt]

I have a couple comments, but I'm also wondering how we can add tests for this. Perhaps should we put an issue into KTF to add plug-in utilities that would help us better test things like this?

[rainest]

I have a couple comments, but I'm also wondering how we can add tests for this. Perhaps should we put an issue into KTF to add plug-in utilities that would help us better test things like this?

Effort/benefit ratio is rather skewed towards the former here. Custom entities aren't particularly common and the controller's interactions with them are quite limited. We literally just append some arbitrary user-provided YAML objects to generated configuration and trust that you wrote them properly in DB-less mode and do nothing with them at all in DB-backed mode (we expect that you'll manage them independently--the DB-less functionality exists because it's impossible to manage config independently in that mode). Making them available would require a non-standard image, which feels like a fairly weighty thing to add to KTF for this alone--if we have other reasons for it that'd make more sense.

For this, added some basic unit stuff to confirm that the entity retrieval function does what it should.

[shaneutt]

Thanks for the patches, LGTM 🖖