feat(cloudbuild): support the new frontend system

[tobiasp284]

Hey, I just made a Pull Request!

Adds new frontend system support to the Cloud Build plugin while preserving its legacy frontend system exports.

• adds a default new-system plugin export at /alpha
• registers the Cloud Build API through ApiBlueprint
• contributes Cloud Build entity content and cards through the catalog entity blueprints
• documents package discovery and explicit installation
• handles Cloud Build responses with missing builds or substitutions, and surfaces failed Google API responses
• adds extension registration, rendering, and client regression tests

Follow-up UI migration

The UI changes are intentionally kept out of this PR so that frontend-system wiring and visual changes can be reviewed independently.

Draft follow-up PR #10265 is stacked on this PR. It migrates the Cloud Build components from direct Material UI v4 usage to @backstage/ui, replaces makeStyles with CSS Modules, moves to Remix icons, and removes the plugin's direct Material UI dependencies. Until this PR merges, GitHub will also show this PR's parent commit in the follow-up diff.

Verification

• yarn tsc:full
• yarn lint
• yarn test plugins/cloudbuild --watchAll=false
• yarn build:api-reports plugins/cloudbuild
• verified the package compiles and loads in Portal Studio

✔️ Checklist

• A changeset describing the change and affected packages.
• Added or updated documentation
• Tests for new functionality and regression tests for bug fixes
• Screenshots attached (no visual changes)
• All commits have a Signed-off-by line in the message.