Refactor OAuth2 Provider Code into Dedicated Package

This PR refactors the OAuth2 provider functionality by moving it from the maincoderd package into a dedicatedoauth2provider package. The change improves code organization and maintainability without changing functionality.

Key changes:

• Created a newoauth2provider package to house all OAuth2 provider-related code
• Moved existing OAuth2 provider functionality fromcoderd/identityprovider to the new package
• Refactored handler functions to follow a consistent pattern of returninghttp.HandlerFunc instead of being handlers directly
• Split large files into smaller, more focused files organized by functionality:
  • app_secrets.go - Manages OAuth2 application secrets
  • apps.go - Handles OAuth2 application CRUD operations
  • authorize.go - Implements the authorization flow
  • metadata.go - Provides OAuth2 metadata endpoints
  • registration.go - Handles dynamic client registration
  • revoke.go - Implements token revocation
  • secrets.go - Manages secret generation and validation
  • tokens.go - Handles token issuance and validation

This refactoring improves code organization and makes the OAuth2 provider functionality more maintainable while preserving all existing behavior.