Add redirect_uri option to oidc_providers entries (#18197)

Allows overriding the `redirect_uri` parameter sent to both the
authorization and token endpoints of the IdP. Typically this parameter
is hardcoded to `<public_baseurl>/_synapse/client/oidc/callback`.

Yet it can be useful in certain contexts to allow a different callback
URL. For instance, if you would like to intercept the authorization code
returned from the IdP and do something with it, before eventually
calling Synapse's OIDC callback URL yourself.

This change enables enterprise use cases but does not change the default
behaviour.

---

Best reviewed commit-by-commit.

---------

Co-authored-by: Eric Eastwood <erice@element.io>

docs/usage/configuration/config_documentation.md

@@ -3662,6 +3662,13 @@ Options for each entry include:
    not included in `scopes`. Set to `userinfo_endpoint` to always use the
    userinfo endpoint.
 
+* `redirect_uri`: An optional string, that if set will override the `redirect_uri`
+  parameter sent in the requests to the authorization and token endpoints.
+  Useful if you want to redirect the client to another endpoint as part of the
+  OIDC login. Be aware that the client must then call Synapse's OIDC callback
+  URL (`<public_baseurl>/_synapse/client/oidc/callback`) manually afterwards.
+  Must be a valid URL including scheme and path.
+
 * `additional_authorization_parameters`: String to string dictionary that will be passed as
    additional parameters to the authorization grant URL.