Function build_join_request_document 

pub fn build_join_request_document(
    contract: &DataContract,
    proposal: &Document,
    leader: &Identity,
    input: &JoinRequestInput,
    entropy: Bytes32,
) -> Result<ModerationCharterRequest, Error>

Builds a join request of input.writer for proposal, the submittedCharter document input.submitted_charter_id, whose owner is leader: the message encrypted to the leader’s decryption key bound to submittedCharter from the writer’s encryption key bound to joinRequest, and recipientId, recipientKeyId and senderKeyId set to match.