[WIP] GEP-1619: Backend as attachment point for Session Persistence

[gcs278]

🚧 WIP

WIP TODO:

• Merge GEP-1619: Sync GEP with API Updates #4870, GEP-1619: Refactor SessionPersistence to discriminated union pattern #4873
• Wait for Backend to be Experimental
• Revisit API field names (consider load balancing, session affinity, etc...)
• Get consensus from implementations
• Remove/update all verbiage in the GEP about backendtrafficpolicy and route-inline
• Adapt/remove edge cases for Backend

What type of PR is this?
/kind gep

What this PR does / why we need it:
Move session persistence from BackendTrafficPolicy and route-inline to the Backend resource (GEP-4488). Remove BackendTrafficPolicy and route-inline API sections, update API Attachment Points to only Backend, and deprecate route-inline.

Which issue(s) this PR fixes:
Fixes # N/A

Does this PR introduce a user-facing change?:

TBD

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This pull-request has been approved by: gcs278
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