Store and retrieve sender data to/from backed up (and exported) sessions

(Requires an MSC)

After doing https://github.com/matrix-org/matrix-rust-sdk/issues/3544 we need to store the sender data (mxid, msk, msk_verified info) to the key backup and to key export files.

This task includes a plan to stop (eventually) treating new-enough sessions from backup as legacy if they lack this information.

**Impact**

This is no use until we have authenticated backups.

Once we have authenticated backups AND this, then new devices will no longer have grey shields on messages whose keys were retrieved from backup.

Start with issue 3544 and the required MSC, then trace the key-backup and key-export handling. Done means sender data (mxid, msk, and msk_verified) is stored and restored in both formats, with a plan for treating sufficiently new sessions without it as non-legacy; authenticated backups are a prerequisite.

Written by the indexing model from the issue text.