feat: implement pr assignment workflow

[shivanee-p]

• Removed Blunderbuss Config: Deleted .github/blunderbuss.yml since Blunderbuss does not support organization-membership checks.
• Created .github/workflows/assign-reviewers.yml
  using actions/github-script to run custom assignment logic.
• Googler/Trusted Author Filter: The action only triggers assignments for PRs opened by members of the googlers GitHub organization or whitelisted automation bots: gcf-owl-bot, release-please, dependabot, renovate, and yoshi-code-bot.
• Path Routing Rules (matching CODEOWNERS teams):
  /handwritten/bigquery/ & /handwritten/bigquery-storage/ $\rightarrow$ @googleapis/bigquery-team
  /handwritten/cloud-profiler/ $\rightarrow$ @googleapis/cloud-profiler-team
  /handwritten/storage/ $\rightarrow$ @googleapis/gcs-team
  /handwritten/firestore/ $\rightarrow$ @googleapis/firestore-team
  /handwritten/spanner/ $\rightarrow$ @googleapis/spanner-team
  /handwritten/pubsub/ $\rightarrow$ @googleapis/pubsub-team
  /handwritten/bigtable/ $\rightarrow$ @googleapis/bigtable-team
  /core/packages/google-auth-library-nodejs/ $\rightarrow$ @googleapis/aion-team
• Load-Balancing Fallback: For all other PRs, the workflow queries the active open PRs to calculate workload and requests a review from the least-loaded individual member of @googleapis/cloud-sdk-nodejs-team, while also CC-ing the team itself.

Fixes b/489433875

[gemini-code-assist]

Note

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