[adt-auth] Implement BTP Service Key auth plugin in @abapify/adt-auth

[ThePlenkov]

Implement a new authentication plugin for BTP Service Key inside the @abapify/adt-auth package by migrating the legacy ServiceKeyParser, BTPServiceKey, and UAACredentials types from packages/adt-cli/src/lib/auth-utils.ts.

• Create new plugin in src/plugins/service-key.ts
• Must implement AuthPlugin interface (authenticate(options)):
  • Accept serviceKeyJson: string | object
  • Use ServiceKeyParser to validate and extract credentials
  • Fetch OAuth2 client_credentials token if required (serviceKey.uaa.url/oauth/token)
  • Return either CookieAuthResult or BasicAuthResult depending on extracted credential type
• Ensure plugin is exported in tsdown.config.ts and src/index.ts
• Migrate PKCE/OAuth helpers from oauth-utils.ts to src/utils/pkce.ts
• Remove old files from adt-cli (auth-utils.ts, oauth-utils.ts) once migration complete

Parent Epic: #70