Define dynamic request attributes for Google GenAI LLM inference spans

[michaelsafyan]

Fixes #2053

Changes

Defines a gcp.gen_ai.operation.config.<key> dynamic attribute in support of Google Gen AI SDK instrumentation (see: PR 3374: include more request configuration options...). In support of this change, this also adds a Google-specific variant of the Gen AI docs. We anticipate that this will serve to enable additional documentation and Semantic Conventions for Google-specific elements beyond the dynamic request attributes being documented here.

Merge requirement checklist

• [X] CONTRIBUTING.md guidelines followed.
• [X] 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]
• [N/A] schema-next.yaml updated with changes to existing conventions.