[SSO] Support edit and remove of OIDC clients

[AlexTugarev]

The preliminary UI provided with #14278 lacks the implementation for edit/removal of OIDC clients.

Once the API is in place, it should be feasible to complement the features.

Note: the result still remains "preliminary", it's just to enable SSO.

[AlexTugarev]

Partially tackled with #16868.
AFAIC, the update RPC needs to be implemented in papi.

[stale]

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