Prototype to flesh out ideas for #39

The plan is to implement in matrix-sdk-crypto + multiverse (or switch to EW if that is a problem).

For the prototype we will have these caveats:

• save a key bundle into a single encrypted media file
• ignore the need to delete the file after use.
• initially, ignore races e.g. new messages arriving while we are preparing

The prototype will help us answer these questions:

• Does the idea basically work?
• Does the invitation event need to change?
• What if new messages are sent after creating the media file?
• Are there other races?
• Is it fast enough? Do I get the keys I care about in a reasonable time?
• We should revisit our list of caveats with MSC3061 and check that they are addressed

Tasks

• Encrypted history sharing: design "key bundle" format #2688
• Encrypted history sharing: design to-device message format #2689
• crypto: Add support for building a bundle of room keys, for sharing with other users matrix-org/matrix-rust-sdk#4504
• Encrypted history sharing: encrypt, upload and share keys to encrypted media blob matrix-org/matrix-rust-sdk#4505
• multiverse: support verifying the multiverse client device matrix-org/matrix-rust-sdk#4508
• multiverse: support sending room invites to other users matrix-org/matrix-rust-sdk#4509
• multiverse: support accepting room invites from other users matrix-org/matrix-rust-sdk#4510
• Encrypted history sharing: hook up share & upload to sending room invites matrix-org/matrix-rust-sdk#4511
• Encrypted history sharing: receive to-device messages and stash data somewhere matrix-org/matrix-rust-sdk#4512
• Encrypted history sharing: download and import keys from encrypted key blob matrix-org/matrix-rust-sdk#4513
• Encrypted history sharing: when I am in a room for which I have received details of a key bundle, trigger a download and import matrix-org/matrix-rust-sdk#4514
• History sharing: handle the key bundle arriving *after* the invite matrix-org/matrix-rust-sdk#4926
• Test out the prototype, and check that it addresses the MSC3061 caveats