feat: Add authentication provider registry for multi-platform support (GitHub + Azure DevOps)

[mnriem]

Problem

Authentication is currently handled inline in _fetch_latest_release_tag() with hardcoded GH_TOKEN/GITHUB_TOKEN env var lookups and GitHub-specific Bearer token headers. As we add Azure DevOps support (and potentially GitLab, Bitbucket), this approach won't scale without scattering provider-specific logic across the codebase.

Proposal

After #2331 lands, create src/specify_cli/authentication/ as a self-contained subdirectory with a provider registry, following the same pattern as src/specify_cli/integrations/:

src/specify_cli/authentication/
├── __init__.py          # AUTH_REGISTRY + register_builtins()
├── base.py              # AuthProvider ABC
├── github.py            # GitHubAuth provider
└── azure_devops.py      # AzureDevOpsAuth provider

AuthProvider base class exposes:

• key — provider identifier (e.g., "github", "azure-devops")
• get_token() -> str | None — resolve credentials from env vars / config
• auth_headers() -> dict — provider-specific auth headers
• api_base_url() -> str — API endpoint root
• is_configured() -> bool — whether credentials are available

Scope

• Builds on top of feat(extensions,presets): authenticate GitHub-hosted catalog and download requests with GITHUB_TOKEN/GH_TOKEN #2331 (must land first)
• Refactor GitHub token logic into GitHubAuth
• Add AzureDevOpsAuth with PAT / env var support (AZURE_DEVOPS_PAT, ADO_TOKEN, etc.)
• _fetch_latest_release_tag() delegates to the registry
• Blast area limited to authentication/ subdirectory

Benefits

• Follows existing registry pattern (integrations/)
• Extensible for GitLab, Bitbucket, etc. without touching core CLI
• Testable in isolation