Add passthrough_authorization_parameters support to OIDC configuration

[odelcroi]

Add passthrough_authorization_parameters support to OIDC configuration

This PR adds the passthrough_authorization_parameters option to OIDC configuration, allowing specific query parameters (like login_hint) to be passed from the redirect endpoint to the authorization grant URL.

This enables clients to provide additional context to identity providers during authentication flows.

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[sandhose]

This would be an extension of the spec, which should go through the spec change process. Given that it's unlikely that such a spec change would land, I think it would make sense to instead have a generic option to 'passthrough' specific query parameters.

Something like

oidc_providers:
  - idp_id: …
    passthrough_authorization_parameters:
      - login_hint

And then passthrough any query parameter passed to /_matrix/client/v3/login/sso/redirect to the OIDC authorization request

[odelcroi]

thanks for your inputs, makes sense, I've made the corresponding changes

[sandhose]

Pretty nice and short in the end, thanks for doing this!

[odelcroi]

@sandhose thanks 👍