Use the v2 (hashed) lookup for identity server queries
Fixes https://github.com/vector-im/riot-web/issues/10556
Implements [MSC2134](https://github.com/matrix-org/matrix-doc/pull/2134) with assistance from [MSC2140](https://github.com/matrix-org/matrix-doc/pull/2140) for auth.
Note: this also changes all identity server requests to use JSON as a request body. URL encoded forms were allowed in v1 but deprecated in favour of JSON. v2 APIs do not allow URL encoded forms.
