docs: add RFC for variation value schema validation

[pradhankukiran]

This PR adds an RFC for variation value schema validation.

The RFC proposes a backend-first implementation for #2499, covering:

• v1 scope: enum, regex, and JSON Schema validation
• flag-level schema ownership
• proposed proto/API shape
• MySQL/PostgreSQL persistence
• create/update validation behavior
• scheduled-change validation behavior
• backend error contract for future UI field mapping
• incremental PR plan

Related Issue

Refs #2499

Notes

This PR is docs-only. Backend model/persistence and validation changes will be submitted in follow-up PRs after RFC review.

[cre8ivejp]

@pradhankukiran, hi!
Can you check the comments I sent when you have time? If you have fixed them already, it would be easier to track if you link the commits in the comments, so the reviewers can resolve them.

[pradhankukiran]

oneof comment:
Fixed here: 621e2ef

Reusable schema note + optional description comment:
Fixed here: 0cddc8c

[pradhankukiran]

@cre8ivejp this PR is ready too