EI: Use 3PID capability to show / hide email UI in settings for MSC3824 OIDC-aware

[hughns]

This is a sub item from #7644

When the homeserver is configured in MSC3861 delegated OIDC mode (e.g. to MAS) the user needs to add and remove 3pids via the UI provided by MAS.

As such, Element iOS should check the capability that instructs clients whether or not they can manage 3PIDs: https://spec.matrix.org/v1.7/client-server-api/#m3pid_changes-capability

If the capability is not set to true then the client should hide the UI for managing emails.

n.b. the Identity Server functionality should remain the same as today.