refactor(crypto): Create a MSC4108 submodule for the qr login types

Our current implementation of this QR code data type corresponds to the
data type defined in MSC4108. The data format has been updated a bit in
MSC4833 and thus we'll need to support both formats for a while.

This moves al the MSC4108-specific parts into a separate MSC-specific
submodule.

MSC4108: matrix-org/matrix-spec-proposals#4108

Author: poljar