Encryption key type should be determined based on the connection

[Moopli]

Two agents communicating over a connection should use encryption key types which are:

• Consistent with their chosen envelope format
• Found within their DID doc
• Found within the other party's DID doc

This can be a value determined once, when the connection is created, and cached in the connection record, this can be determined live whenever a message is sent, this can be recalculated whenever a DID doc updates, etc.

This can be used to enforce requirements on key type (eg for regulatory purposes) since an agent which has requirements can refrain from presenting non-compliant key types on their DID docs.