Support SSSS sharing of cross signing master key

We now want to transfer the cross-signing master key over SSSS sharing too, such that we can operate without SSSS storage at all.

 * Request the master key along with other cross-signing private key parts
 * Allow the master key to be shared

Related to https://github.com/vector-im/riot-web/issues/13895
Related to https://github.com/vector-im/riot-web/issues/13825