Add grouping policies to server response for RBAC multi-role support

[Copilot]

Casbin.js already supports users with multiple roles via casbin-core's RBAC enforcer, but the test server wasn't returning grouping policies (g rules), making it appear unsupported.

Changes

• Server: Modified CasbinService.getEnforcerConfig() to include grouping policies alongside regular policies in API responses
• Tests: Added test case demonstrating multi-role scenarios (user with both admin and editor roles)
• Documentation: Added RBAC section to README with backend response format example

Backend Response Format

Backends using auto mode must return both policy and grouping rules:

{
  "m": "<model configuration>",
  "p": [
    ["p", "admin", "data1", "read"],
    ["p", "editor", "data2", "read"],
    ["g", "alice", "admin"],    // Role assignment
    ["g", "alice", "editor"]    // Multiple roles per user
  ]
}

This establishes the correct pattern for implementing Casbin backends that support multiple roles per user.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[feature] Is Casbin.js not supported for scenarios where a user has multiple roles, meaning it hasn’t been adapted/implemented yet?</issue_title>
<issue_description>required by: casbin/casbin#1295 (comment)

@jayddzjh</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [feature] Is Casbin.js not supported for scenarios where a user has multiple roles, meaning it hasn’t been adapted/implemented yet? #301

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