Add Session Persistence support for NGINX OSS and Plus users

[salonichf5]

Proposed changes

Write a clear and concise description that helps reviewers understand the purpose and impact of your changes. Use the
following format:

Problem: Users need session persistence (“sticky sessions”) so traffic from the same client consistently routes to the same upstream instance for stateful apps.

Solution: Adds two types of session persistence: IP-based persistence by allowing users to set the ip_hash load-balancing method via the Upstream Settings Policy, and cookie-based persistence by adding support for sessionPersistence configuration at the Route level to enable sticky sessions using a sticky cookie. Cookie-based persistence is available only to NGINX Plus users and is supported in experimental mode. Additionally, the PR expands load-balancing support to include all methods available in both NGINX OSS and NGINX Plus.

Testing: Ensure pipeline succeeds. Functional test added.

Please focus on (optional): If you any specific areas where you would like reviewers to focus their attention or provide
specific feedback, add them here.

Closes #2676

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 (update on doc side)
• 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.

Adds session persistence support for NGINX OSS and NGINX Plus. OSS and Plus users can configure `ip_hash` via `UpstreamSettingsPolicy` to provide client IP–based session affinity, while NGINX Plus users can configure `sessionPersistence` on `HTTPRoute` and `GRPCRoute` to enable cookie-based session persistence. Cookie-based session persistence is available only to NGINX Plus users and is currently in experimental mode.

[codecov]

Codecov Report

❌ Patch coverage is 92.01102% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.36%. Comparing base (5824970) to head (0f2ed98).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/controller/manager.go	0.00%	9 Missing ⚠️
internal/controller/state/graph/route_common.go	93.75%	4 Missing and 2 partials ⚠️
...ginx/config/policies/upstreamsettings/validator.go	88.88%	3 Missing and 2 partials ⚠️
...ernal/controller/nginx/config/validation/common.go	88.23%	3 Missing and 1 partial ⚠️
internal/controller/nginx/config/upstreams.go	93.75%	1 Missing and 1 partial ⚠️
internal/controller/state/graph/backend_refs.go	71.42%	1 Missing and 1 partial ⚠️
internal/controller/state/validation/validator.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4471      +/-   ##
==========================================
+ Coverage   86.25%   86.36%   +0.11%     
==========================================
  Files         132      132              
  Lines       14560    14849     +289     
  Branches       35       35              
==========================================
+ Hits        12558    12824     +266     
- Misses       1790     1806      +16     
- Partials      212      219       +7     

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[sjberman]

Can you make the release note a bit more descriptive. It's okay if it's a few sentences. I'd like to be clear about what we support for each OSS and Plus.

[sjberman]

Do you also mind updating the PR description (and commit message) to also add some more detail on the two features that are added as part of this (LB methods and sessionPersistence support).