Add support for ForwardAuth protected _synapse/admin API via CORS withCredentials option.

[jwmay2012]

I have my matrix.domain.com/_synapse/admin protected behind Authelia. It would be nice to allow me to keep that protection and authenticate with Authelia on these CORS requests by using the withCredentials = true option.
This allows the Cookie header to be sent along with the CORS requests to the matrix API, authenticating with Authelia (or any other Forward Auth).

https://stackoverflow.com/questions/54718199/cookie-based-authentication-via-rest-api-in-react-admin

Should be as simple as including
options.credentials = 'include'
https://github.com/Awesome-Technologies/synapse-admin/blob/master/src/synapse/dataProvider.ts#L16
and
credentials: 'include'
https://github.com/Awesome-Technologies/synapse-admin/blob/master/src/synapse/authProvider.ts#L20

Perhaps with a config flag to optionally switch it on.

Great project, thanks :)

[aine-etke]

https://github.com/etkecc/synapse-admin/blob/main/docs/cors-credentials.md