feat(gcp): add gcp-monitoring plugin manifest (IW-216, IW-217)

[eyriehq-bot]

Summary

• Adds plugins/oss/gcp-monitoring/plugin.json declaring gcp_scopes and gcp_roles
• Moves OAuth scope management from hardcoded setdefault in gcp_sts_service.py to plugin manifest
• Sets the pattern for all upcoming GCP plugins (IW-218–224)

Part of

release/0.14.0 cycle (2 May – 15 May 2026)

[mguptahub]

Closing — all content from this PR is already on release/0.14.0. The gcp-monitoring/plugin.json was added directly to the release branch (commit b39e8be), and the README/logo changes landed via PRs #24, #25, #26. No new changes to merge; the conflict is because the release branch moved ahead of this branch's base. IW-216/IW-217 monitoring manifest is live.