[adt-auth] Support COPILOT_ADT_SERVICE_KEY / ADT_SERVICE_KEY env var injection

[ThePlenkov]

Implement a utility in @abapify/adt-auth to support reading the BTP service key for authentication from COPILOT_ADT_SERVICE_KEY and ADT_SERVICE_KEY environment variables.

• Utility should parse (if needed) a JSON service key and return a ready Destination config for the service-key plugin
• Should be callable both from CLI and programmatic flows

Parent Epic: #70
Blocks: #migrate-service-key-auth-plugin