History sharing: store details of the provenance of a shared megolm session

[richvdh]

If Bob has forwarded us a megolm session, we'll need to present it as "as presented by Bob" in the UI. The first step is to add this data to the InboundGroupSession in the crypto store.

• feat(crypto): Add forwarder_data to InboundGroupSession and pickle. #5943
• feat: Add forwarder and forwarder_device to EncryptionInfo. #5945

Part of element-hq/element-meta#2829

[kaylendog]

Quite similar to #3556, might be worth discussing