Add OAuth 2.1 authentication support

[EhabY]

Implements OAuth 2.1 with PKCE as an alternative authentication method to session tokens. When connecting to a Coder deployment that supports OAuth, users can choose between OAuth and legacy token authentication.

Key changes:

OAuth Flow:

• Add OAuthSessionManager to handle the complete OAuth lifecycle: dynamic client registration, PKCE authorization flow, token exchange, automatic refresh, and revocation
• Add OAuthMetadataClient to discover and validate OAuth server metadata from the well-known endpoint, ensuring server meets OAuth 2.1 requirements
• Handle OAuth callbacks via vscode:// URI handler with cross-window support for when callback arrives in a different VS Code window

Token Management:

• Store OAuth tokens (access, refresh, expiry) per-deployment in secrets
• Store dynamic client registrations per-deployment in secrets
• Proactive token refresh when approaching expiry (via response interceptor)
• Reactive token refresh on 401 responses with automatic request retry
• Handle OAuth errors (invalid_grant, invalid_client) by prompting for re-authentication

Integration:

• Add auth method selection prompt when server supports OAuth
• Attach OAuth interceptors to CoderApi for automatic token refresh
• Clear OAuth state when user explicitly chooses token auth
• DeploymentManager coordinates OAuth session state with deployment changes

Error Handling:

• Typed OAuth error classes (InvalidGrantError, InvalidClientError, etc.)
• Parse OAuth error responses from token endpoint
• Show re-authentication modal for errors requiring user action