Support for v2 of MSC3903 (#3155)
* v2 of MSC3903 implementation
This is a deliberate breaking change on an unstable feature.
* Test correct protocol version
* Fix up test
* v2 of MSC3903 implementation
This is a deliberate breaking change on an unstable feature.
* Test correct protocol version
* Fix up test
* Reinstate v1 support to make this a non-breaking change
Deprecates several experimental types
