Propose semantic conventions for GCP client library identification

[michaelsafyan]

Propose semantic conventions for GCP client library identification and attribution.

Changes

Defines a set of gcp.client.* scope attributes for identifying Google Cloud client libraries.

Merge requirement checklist

• ✅ CONTRIBUTING.md guidelines followed.
• ✅ Change log entry added, according to the guidelines in When to add a changelog entry.
• N/A: schema-next.yaml updated with changes to existing conventions.