Get room by name end point

[thejhh]

GET /_matrix/client/r0/directory/room/{roomAlias}

Should be updated to use newer version. However, remember to refactor our SimpleMatrixClient to use it also at fi.hg.matrix.

See also:

• https://spec.matrix.org/v1.2/client-server-api/#get_matrixclientv3directoryroomroomalias