feat: [Superfed] go-auth0 SDK Support

[developerkunal]

🔧 Changes

This PR adds support for the Express Configuration feature (Superfed/OIN integration) to the go-auth0 SDK.

Types and methods added:

• Added ExpressConfiguration field to Client struct for OIN integration support
• Added IsSystem field to ClientGrant struct to identify system-managed grants
• Added IsSystem field to ResourceServer struct to identify system-managed APIs
• Generated getter methods for all new fields: GetExpressConfiguration(), GetIsSystem()

Functionality:

• Clients can now be created with app_type: "express_configuration" for OIN apps
• Regular web clients can include the express_configuration object for SSO/SCIM setup
• Client grants and resource servers with is_system: true are properly identified as system-managed
• Special handling for urn:auth0:express-configure resource server with predefined template

📚 References

• Jira: SDK-7039
• Feature: Okta Integration Network (OIN) Express Configuration
• Related: Superfed authentication and provisioning capabilities

🔬 Testing

All functionality is covered by comprehensive tests:

1. TestClient_ExpressConfiguration (4 tests):

   • Create Express Configuration client
   • Read Express Configuration client
   • Update regular web client with express_configuration object
   • Verify express_configuration object is preserved

2. TestClientGrant_IsSystem (2 tests):

   • Verify user-created grants have is_system=false
   • Verify system grants are properly identified

3. TestClientGrant_ExpressConfiguration (2 tests):

   • Verify Express Config clients automatically receive system grant
   • Verify system grant has is_system=true

4. TestResourceServer_ExpressConfiguration (4 tests):

   • Create urn:auth0:express-configure resource server
   • Read and verify predefined template values
   • Verify update attempts are rejected (API enforces immutability)
   • Verify resource server appears in list with correct properties

HTTP recordings captured for all tests (playback mode tested successfully).

📝 Checklist

• All new/changed/fixed functionality is covered by tests
• I have added documentation for all new/changed functionality

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.70%. Comparing base (1c9c68b) to head (3d7c086).

Additional details and impacted files

@@            Coverage Diff             @@
##               v1     #652      +/-   ##
==========================================
+ Coverage   96.68%   96.70%   +0.01%     
==========================================
  Files          62       62              
  Lines       10654    10710      +56     
==========================================
+ Hits        10301    10357      +56     
  Misses        235      235              
  Partials      118      118              

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