feat:Add Auth0-Custom-Domain header support for Multiple Custom Domains (MCD)

[kishore7snehil]

Added support for the Auth0-Custom-Domain header to enable Multiple Custom Domains (MCD).

Changes

• Added custom_domain parameter to ManagementClient and AsyncManagementClient for setting the Auth0-Custom-Domain header globally
• Added CustomDomainHeader helper for per-request header override via request_options
• Implemented whitelist enforcement via httpx event hooks , header is only sent on 8 whitelisted endpoints, stripped on all others

Path	Description
/api/v2/jobs/verification-email	Verification email jobs
/api/v2/tickets/email-verification	Email verification tickets
/api/v2/tickets/password-change	Password change tickets
/api/v2/organizations/{id}/invitations	Organization invitations
/api/v2/users	Users (list/create)
/api/v2/users/{id}	Users (get/update/delete)
/api/v2/guardian/enrollments/ticket	Guardian enrollment tickets
/api/v2/self-service-profiles/{id}/sso-ticket	SSO tickets

Enforcement is implemented via httpx request event hooks that strip the header on non-whitelisted paths before the request is sent.

• Per-request CustomDomainHeader takes precedence over the global custom_domain when both are provided
• Added 22 unit tests covering initialization, header merging, whitelist enforcement, and async support
• Updated README.md with Custom Domains documentation

Usage

from auth0.management import ManagementClient, CustomDomainHeader

# Global — header sent on all whitelisted endpoints
client = ManagementClient(
    domain="your-tenant.auth0.com",
    token="YOUR_TOKEN",
    custom_domain="login.mycompany.com",
)

# Per-request override
client.users.create(
    connection="Username-Password-Authentication",
    email="user@example.com",
    password="SecurePass123!",
    request_options=CustomDomainHeader("other.mycompany.com"),
)

References

• Closes #794

Checklist

• All new/changed/fixed functionality is covered by tests
• I have added documentation for all new/changed functionality
• No Fern-generated files modified (all changes in .fernignore-listed files)

[duedares-rvj]

LGTM