Add API SAML identifier update option for domain change

What does this MR do?

Unlinking and relinking an account can be very disruptive, so the recommended method of updating the identifier is to use the API.

If the identifier is not changing, there's no reason to go through the process, so it's only required if the email domain is in the NameID/identifier.

Related issues

Numerous support engineers have asked about this recently, including in relation to these tickets (internal):

1. https://gitlab.zendesk.com/agent/tickets/373448
2. https://gitlab.zendesk.com/agent/tickets/345353

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