Required MSC2966 contacts field is missing from OIDC client registration

[hughns]

MSC2966 defines how Matrix clients show use dynamic client registration.

The contacts field is required but missing from EIX. Note that the RFC7591 doesn't make it mandatory which might be causing some confusion.

Simple fix is to add a value like ["[email protected]"] which can be overridden like the other OIDC registration values.