[adt-cli] Add BTP service key option to CLI interactive login command

[ThePlenkov]

Update packages/adt-cli/src/lib/commands/auth/login.ts to:

• Add @abapify/adt-auth/plugins/service-key as second choice in promptManualConfig() next to Basic Auth
• Auto-detect ADT_SERVICE_KEY or COPILOT_ADT_SERVICE_KEY env vars and use for non-interactive login
• Add --service-key <path|json> CLI flag for alternative input
• Ensure CLI fallback, error handling and documentation are correct

Parent Epic: #70
Blocks: #inject-service-key-env