feat:Add Auth0-Custom-Domain header support for Multiple Custom Domains (MCD) (#799)

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

```python
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](#794)

### Checklist

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

Author: kishore7snehil

README.md

@@ -315,6 +315,44 @@ client = Auth0(
 )
 ```
 
+### Custom Domains
+
+If your Auth0 tenant uses multiple custom domains, you can specify which custom domain to use via the `Auth0-Custom-Domain` header. The SDK enforces a whitelist, the header is only sent on supported endpoints.
+
+**Global (all whitelisted requests):**
+
+```python
+from auth0.management import ManagementClient
+
+client = ManagementClient(
+    domain="your-tenant.auth0.com",
+    token="YOUR_TOKEN",
+    custom_domain="login.mycompany.com",
+)
+```
+
+**Per-request override:**
+
+```python
+from auth0.management import ManagementClient, CustomDomainHeader
+
+client = ManagementClient(
+    domain="your-tenant.auth0.com",
+    token="YOUR_TOKEN",
+    custom_domain="login.mycompany.com",
+)
+
+# Override the global custom domain for this specific request
+client.users.create(
+    connection="Username-Password-Authentication",
+    email="user@example.com",
+    password="SecurePass123!",
+    request_options=CustomDomainHeader("other.mycompany.com"),
+)
+```
+
+If both a global `custom_domain` and a per-request `CustomDomainHeader` are provided, the per-request value takes precedence.
+
 ## Feedback
 
 ### Contributing