feat: add playwright-config input to frontend-e2e-against-stack action

[srsholmes]

Summary

Adds a new optional playwright-config input to actions/plugins/frontend-e2e-against-stack so callers can point the action at a specific Playwright config when their repo maintains more than one.

- uses: grafana/plugin-ci-workflows/actions/plugins/frontend-e2e-against-stack@…
  with:
    stack-slug: my-stack
    env: dev-central
    playwright-config: e2e/assistant/playwright.config.ts

Why

Today the action runs `npx playwright test` unconditionally, which falls back to Playwright's own resolution (playwright.config.ts in the working directory). That's fine for the single-config case but leaves no escape hatch when a repo has more than one Playwright config — typically when a stack-based integration suite has different workers / timeouts / project settings from the main e2e suite.

Backwards compatibility

Empty default preserves current behaviour — existing callers see no change.

Test plan

• Verify the action still runs cleanly for an existing caller that does NOT set playwright-config (e.g. via a release-please preview or a manual workflow_dispatch in a consumer repo)
• Verify a caller that DOES set playwright-config: path/to/custom.config.ts runs against that config
• Marking as Draft until at least one in-repo verification run — happy to take direction on how this project typically validates action changes (mirror PR in a consumer repo?)

🤖 Generated with Claude Code

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[yduartep]

Nice!!