Add native GitHub auth mode with default token flow and GitHub App support

[Copilot]

This change adds a dedicated GitHub authentication type to the Infinity datasource, with token auth as the default and optional GitHub App-based auth. It replaces the prior guided GitHub setup with first-class config + backend handling for both flows.

• Auth model and config surface

  • Added auth_method: "github" to frontend/backend auth enums.
  • Added GitHub-specific config payload (authType, appId, installationId, apiUrl) and secure fields (githubToken, githubAppPrivateKey).

• UI changes (datasource config)

  • Added a new GitHub auth card in the main auth selector.
  • Introduced GitHub auth editor with:
    • Token auth (default): PAT input
    • GitHub App auth: App ID, Installation ID, API URL, private key
  • Removed GitHub from “Other Auth Providers” so GitHub is now configured through the primary auth path.

• Backend auth behavior

  • Token mode now applies `Authorization: ****** via existing bearer header path.
  • App mode adds transport-level GitHub App token acquisition:
    • Signs app JWT with the configured private key
    • Calls POST /app/installations/{installation_id}/access_tokens
    • Caches/reuses installation tokens until near expiry
    • Injects bearer token per request

• Validation and docs alignment

  • Added GitHub auth validation rules (token required for token mode; app credentials + private key required for app mode).
  • Updated GitHub example docs to use the new GitHub auth type and note App auth availability.

jsonData:
  auth_method: github
  github:
    authType: token   # default
  allowedHosts:
    - https://api.github.com
secureJsonData:
  githubToken: ${GITHUB_PAT}