Define dynamic request attributes for Google GenAI LLM inference spans

[Rima-ag]

Addresses comments on #2125

Copied Description from original PR

Fixes open-telemetry/semantic-conventions-genai#27

Changes

Defines a gcp.gen_ai.operation.config. dynamic attribute in support of Google Gen AI SDK instrumentation (see: open-telemetry/opentelemetry-python-contrib#3374). 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

• 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]
    *[N/A] schema-next.yaml updated with changes to existing conventions.

[github-actions]

This PR contains changes to area(s) that do not have an active SIG/project and will be auto-closed:

• gcp

Such changes may be rejected or put on hold until a new SIG/project is established.

Please refer to the Semantic Convention Areas
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