Add support for overriding `id_token_signing_alg_values_supported` for an OpenID identity provider (#18177)

Normally, when `discovery` is enabled,
`id_token_signing_alg_values_supported` comes from the OpenID Discovery
Document (`/.well-known/openid-configuration`). If nothing was
specified, we default to supporting `RS256` in the downstream usage.

This PR just adds support for adding a default/overriding the the
discovered value [just like we do for other things like the
`token_endpoint`](https://github.com/element-hq/synapse/blob/1525a3b4d48a0f5657d61423e1f205bff9a77948/docs/usage/configuration/config_documentation.md#oidc_providers),
etc.