Element R: support for Key backup

[richvdh]

Depends on #25315

https://docs.google.com/document/d/18LuSmlaoeZpsOsiPKIdAxKYsbr9V1etMAYm2vxMId7A/edit#heading=h.wf09j8nuwlov

• https://github.com/vector-im/crypto-internal/issues/87
• https://github.com/vector-im/crypto-internal/issues/100
• https://github.com/vector-im/crypto-internal/issues/101
• https://github.com/vector-im/crypto-internal/issues/102
• https://github.com/vector-im/crypto-internal/issues/103
• https://github.com/vector-im/crypto-internal/issues/104
• https://github.com/vector-im/crypto-internal/issues/105
• https://github.com/vector-im/crypto-internal/issues/106
• https://github.com/vector-im/crypto-internal/issues/107
• https://github.com/vector-im/crypto-internal/issues/108
• https://github.com/vector-im/crypto-internal/issues/109
• https://github.com/vector-im/crypto-internal/issues/110
• https://github.com/vector-im/crypto-internal/issues/111
• https://github.com/vector-im/crypto-internal/issues/112
• https://github.com/vector-im/crypto-internal/issues/113
• https://github.com/vector-im/crypto-internal/issues/114
• https://github.com/vector-im/crypto-internal/issues/115
• https://github.com/vector-im/crypto-internal/issues/155
• #26259

[richvdh]

Basic support for key backup is done, though there remain a couple of bugs (eg #26316).