Introduce gen_ai.request.seed as needed by cohere

[xrmx]

Changes

And deprecate gen_ai.openai.request_seed.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.

[karthikscale3]

good call

[codefromthecrypt]

thanks, the less one-offs the better!