Send id_access_token to HS for use in proxied IS requests
This passes along the `id_access_token` to the HS, which it will need when
speaking v2 IS APIs to the IS.
Unfortunately, some HSes seem to explode when given this new parameter, so we
only pass it along for the moment if an unstable feature `m.id_access_token` is
also set.
Part of https://github.com/vector-im/riot-web/issues/10525
Defined in MSC2140
