crypto: Add support for building an encrypted blob of room keys, for sharing with other users

[richvdh]

Our proposed solution to sharing encrypted room history on invite involves uploading all of the message keys for a given room to an encrypted media blob.

This issue covers the addition of an API to matrix-sdk-crypto which will create such a blob. We expect that the work of actually uploading the blob, and sharing the keys with other clients, will be handled separately (and is therefore tracked in another issue).

The format of this encrypted blob is to be determined by element-hq/element-meta#2688.

Part of element-hq/element-meta#2685.