feat: Add OAuth 2.0 Authentication Support in Edit Gateway

[shams858]

Title: Add OAuth 2.0 Authentication Support in Edit Gateway

Description:
This PR implements OAuth 2.0 authentication support for editing gateways in the MCP Gateway admin interface.

Changes Made

Frontend Updates

• Edit Gateway Modal: Added OAuth 2.0 option to Authentication Type dropdown
• OAuth Form Fields: Implemented complete OAuth 2.0 configuration form including:
  • Grant Type selection (Client Credentials vs Authorization Code)
  • Client ID and Client Secret fields
  • Token URL configuration
  • Authorization Code specific fields (Authorization URL, Redirect URI, Scopes)
  • Dynamic field visibility based on grant type selection
• JavaScript Handlers: Added OAuth field population, validation, and form submission handling
• Security: Client secrets are never populated in edit forms for security

Backend Updates

• Schema Enhancement: Added oauth_config field to GatewayUpdate schema
• Admin API: Updated admin_edit_gateway endpoint to handle OAuth configuration
• Gateway Service: Enhanced update_gateway method to process OAuth updates
• Encryption: Integrated OAuth client secret encryption for secure storage

Security Features

• Client secrets are encrypted before database storage
• Client secrets are never returned in API responses
• Edit forms show empty secret fields (users must re-enter to change)
• Proper validation and sanitization of OAuth configuration data

Related Issues

Related to this #782

Screenshots

[crivetimihai]

Awesome, thank you. Reviewing and rebase it, please don't push for now, merge in progress!

[crivetimihai]

Ready to merge!

[crivetimihai]

Awesome, thank you!