feat: extend load resources to support multiple gatewayclasses and envoyproxies

[shawnh2]

What type of PR is this?

What this PR does / why we need it:

Change the load function in gatewayapi/resources to support multiple gatewayclasses and envoyproxies:

• For file provider, it can prevent resources from being lumped together (#5858)
• For egctl x translate, allow it to process multiple gatewayclasses tranlaste and even merge gateways translate (#3937)

Which issue(s) this PR fixes:

Fixes #3207, close #3960

Release Notes: Yes

[arkodg]

is this for v1.4.0 @shawnh2 ?

[shawnh2]

is this for v1.4.0 @shawnh2 ?

yeah

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 79.85075% with 27 lines in your changes missing coverage. Please review.

Project coverage is 65.93%. Comparing base (da98c66) to head (8750b75). Report is 246 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/provider/file/store.go	29.62%	15 Missing and 4 partials :warning:
internal/cmd/egctl/simple_controller.go	68.75%	3 Missing and 2 partials :warning:
internal/cmd/egctl/translate.go	78.57%	2 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5950      +/-   ##
==========================================
+ Coverage   65.82%   65.93%   +0.10%     
==========================================
  Files         217      218       +1     
  Lines       36016    36091      +75     
==========================================
+ Hits        23709    23796      +87     
+ Misses      10832    10812      -20     
- Partials     1475     1483       +8     

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[github-actions[bot]]

This pull request has been automatically marked as stale because it has not had activity in the last 30 days. Please feel free to give a status update now, ping for review, when it's ready. Thank you for your contributions!

[github-actions[bot]]

This pull request has been automatically marked as stale because it has not had activity in the last 30 days. Please feel free to give a status update now, ping for review, when it's ready. Thank you for your contributions!

[shawnh2]

Can we land this in v1.5? I'm working on enable standalone for all e2e tests recently, and this will block me.

[shawnh2]

too much of conflicts, will reopen a PR for this