Support custom CRDs for ExtensionServer in Standalone Mode

[daum3ns]

This change enables the file-based (standalone) provider to understand and load custom resources that are defined by an extension server.

• the LoadResourcesFromYAMLBytes now accepts a server config pointer. When a resource kind is unknown, the loader checks the configured ExtensionManager and, if the GVK matches a declared policy resource, stores the object as an ExtensionServerPolicy
  • Updated callers to pass the new parameter
• the newOfflineGatewayAPIClient function now accepts extensionServerPoliciesGVKs, which are then registered in the scheme. this allows the offline reconciler to manipulate custom resources
• added testcases for decoder, offline_controller and config_loader,
  • the config reload test starts Envoy Gateway in standalone mode, hot-reloads a configuration that adds an ExtensionManager with policy resources, and verifies the new GVKs are recognized

Fixes #7141

Release Notes: Yes/No

Notes:
I found out that we most probably have the same problem with ExtensionManagers backendResources and resources field (at least the config_loader test can easily be extended to show that the registration in the scheme is missing).

I think it could be fixed in a similar way (although not sure whether the translator needs to be adapted as well). See the commented code snippets...

However, I'm very new to envoy-gateway so I'm still raising this PR to get a first, much appreciated feedback whether it makes sense to continue in this direction..

[codecov]

Codecov Report

❌ Patch coverage is 86.36364% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.00%. Comparing base (fa9f9c3) to head (d66478e).
⚠️ Report is 33 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/provider/file/store.go	40.00%	2 Missing and 1 partial ⚠️
internal/gatewayapi/runner/runner.go	0.00%	2 Missing ⚠️
internal/provider/file/file.go	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7331      +/-   ##
==========================================
+ Coverage   70.92%   72.00%   +1.07%     
==========================================
  Files         229      230       +1     
  Lines       41071    33372    -7699     
==========================================
- Hits        29131    24030    -5101     
+ Misses      10221     7592    -2629     
- Partials     1719     1750      +31     

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[daum3ns]

@shawnh2 i have added a test scenario which shows we have the same problem for the other 2 resources fields in the extension manager in standalone mode....
f996f5a

i think i will address this in this PR as well.. (or would you prefer a separate Issue/PR ?)
Can you answer this question here?
https://github.com/daum3ns/envoy-gateway/blob/support-custom-CRDs-for-ExtensionServer-in-Standalone-Mode/internal/gatewayapi/resource/load.go#L454
Im not sure whether all extensions should go in the same field or wether there are dedicated fields for the different types...