feat: populate custom fields from Backstage entity annotations

[kwent]

Summary

• Add annotation-to-attribute passthrough for all 4 entity types (services, functionalities, teams, catalog entities)
• Add annotation-to-catalog-property mapping for services and catalog entities
• Two new annotation prefixes: rootly.com/<entity>-attr-<key> for API attributes, rootly.com/<entity>-property-<slug> for catalog properties
• Value coercion: booleans, JSON arrays/objects, plain strings
• Hardcoded fields (name, backstage_id, etc.) always override passthrough — no control-plane leakage

Example usage

annotations:
  rootly.com/service-attr-color: "#FF5733"
  rootly.com/service-attr-notify_emails: '["alice@co.com","bob@co.com"]'
  rootly.com/service-attr-show_uptime: "true"
  rootly.com/service-property-my-custom-field: "value"

Changes

• constants.ts — 6 new annotation prefix constants
• api.ts — extractAnnotationEntries, extractPassthroughAttributes, extractProperties functions; integrated into all 8 import/update methods
• api.test.ts — 17 unit tests

Dependencies

• Rootly API: requires slug support for catalog_property_id in catalog_fields_support.rb (separate PR)
• No changes needed in backstage-plugin (frontend) or backstage-plugin-entity-processor (backend) — they pass full entities through

Test plan

• 17 unit tests for extraction functions (all passing)
• Integration test: add annotations to test Backstage entity, trigger entity processor sync, verify Rootly service has custom attributes + properties
• Verify existing sync still works (no regressions on name/description/pagerduty_id)

Ref: PRF-2548

[kwent]

@greptileai