Implement ProxySettingsPolicy

[ciarams87]

Proposed changes

Problem: We need to implement the ProxySettingsPolicy API

Solution: Implement ProxySettingsPolicy

Manual Testing: Tested using the example files provided which will be the basis of the documentation:
Steps (using the provided examples):

• Deploy an application which returns a large payload (coffee) and one which does not (tea)
• Apply the PSP for the Gateway which will support the smaller payload application
• Verify tea works but coffee does not (response too large error in NGINX logs)
• Apply the PSP for the coffee route which increases the proxy buffer size
• Verify coffee now works (note: return output to file if testing this as it fills the terminal)

Also tested the validation works as expected.

Note that the Programmed status implementation, CEL, and functional testing will all be done in follow up PRs

Closes #4261

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

[sjberman]

Nice work!

[shaun-nx]

Does PolicyAffectedReason only apply to these three policies?

[ciarams87]

Currently yes, as these are the only custom Policies we have, but any new Policies should also ensure there is PolicyAffectedReason applied to the affected resource the Policy is targeting.

[bjee19]

nice