Proposal to have the appservice registration `type` be optional

[turt2live]

Documentation: https://docs.google.com/document/d/1ng01UlGynBfktSepGHtsCArsE5ZM6mOZzZn-ULtTZEM/edit?usp=sharing
Related/resolves: #1312

[Half-Shot]

We've since made synapse and dendrite (and all the matrix-org, maunium and other bridges) support type, so I think there is very little point in driving the other way now. Can we close this one @turt2live ?