mgmt: Adds Auth0-Custom-Header on allow-listed Management API endpoints

[kailash-b]

Changes

Adds support for the Auth0-Custom-Domain header on Management API endpoints that generate user-facing links. Auth0 tenants with Multiple Custom Domains (MCD) enabled must supply this header so that generated
URLs (verification emails, password reset tickets, invitations, etc.) resolve against the correct custom domain.
Affected endpoints (allow-list):

• POST /api/v2/tickets/email-verification
• POST /api/v2/tickets/password-change
• POST /api/v2/organizations/{id}/invitations
• POST /api/v2/guardian/enrollments/ticket
• POST /api/v2/jobs/verification-email
• POST /api/v2/jobs/users-imports
• POST /api/v2/users and PATCH /api/v2/users/{id}
• POST /api/v2/self-service-profiles/{id}/sso-ticket

New classes/APIs:

• CustomDomainInterceptor (DelegatingHandler) — strips Auth0-Custom-Domain from any request whose path is not on the allow-list, preventing the header from leaking to unrelated endpoints.
• CustomDomainHeader.For(string domain) — convenience factory returning a RequestOptions pre-populated with the Auth0-Custom-Domain header for per-request use.
• CustomDomain property added to ClientOptions and ManagementClientOptions — when set (and no custom HttpClient is provided), the SDK automatically wires up CustomDomainInterceptor.

Usage summary:

// Option 1 — Global via ManagementClient (recommended)
var client = new ManagementClient(new ManagementClientOptions
{
    Domain = "my.auth0.domain",
    TokenProvider = ...,
    CustomDomain = "login.mycompany.com"   // interceptor injected automatically
});

// Option 2 — Per-request override
await client.Tickets.VerifyEmailAsync(request,
    CustomDomainHeader.For("login.mycompany.com"));

// Option 3 — Manual via ManagementApiClient (bring-your-own token)
var client = new ManagementApiClient("token", new ClientOptions
{
    BaseUrl = "https://my.auth0.domain/api/v2",
    CustomDomain = "login.mycompany.com",
    HttpClient = new HttpClient(new CustomDomainInterceptor())   // required when BYO HttpClient
});

• Examples.md updated with a new section 5 covering all three patterns.

References

Testing

• This change adds unit test coverage

• This change adds integration test coverage

• This change has been tested on the latest version of the platform/language or why not

Checklist

• I have read the Auth0 general contribution guidelines

• I have read the Auth0 Code of Conduct

[tanya732]

Fix indentation, if needed