Add register-specific request token endpoint
Add register-specific request token endpoint

As per https://github.com/matrix-org/matrix-doc/pull/343

williamboman/matrix-js-sdk,matrix-org/matrix-js-sdk,krombel/matrix-js-sdk,williamboman/matrix-js-sdk,matrix-org/matrix-js-sdk,williamboman/matrix-js-sdk,krombel/matrix-js-sdk,krombel/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk