Encrypted appservices checklist

[turt2live]

• All of E2EE Checklist #134
• Synapse implementation of MSC3202: Encrypted appservices matrix-org/matrix-spec-proposals#3202
• Wire up appservice side of SDK to the MatrixClient/CryptoClient bits

[turt2live]

Synapse side blocked on matrix-org/synapse#10653

[turt2live]

Closing in favour of more detailed individual issues opened on the repo.