Restore authorization via BTP service key for unified cloud/on-prem CLI compatibility

[ThePlenkov]

Background

In the past, the CLI supported both on-prem (basic authentication) and SAP BTP using a service key for authentication. Recent updates have dropped or broken this feature, yet we need to support both scenarios for modern hybrid ABAP landscapes.

Request

• Restore authentication support using BTP service keys (via the new ADT_SERVICE_KEY/COPILOT_ADT_SERVICE_KEY environment variables)
• Ensure seamless fallback/selection between:
  • On-prem with basic authentication
  • Cloud via BTP service key (using extracted credentials)
• Update CLI and related packages to fully support both scenarios again
• Update documentation/add usage examples
• CI must exercise both auth modes (with secret injection)

Notes

• Relevant env vars are: COPILOT_ADT_SERVICE_KEY for Copilot, ADT_SERVICE_KEY for CI Actions
• Please review previous implementation for reference and avoid regressions

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This will enable fully compatible authentication for both on-prem and BTP cloud ABAP systems via the CLI.