[MVP] Implement member log including accesscontroller

[aeddi]

Depends on #1461
Implement member log with its accesscontroller.

message MemberEntryEnvelope {
  bytes encrypted_member_payload = 1;
  bytes member_payload_signature = 2;  // Signed by member_device_priv_key
}

message MemberEntryPayload {
  bytes member_pub_key = 1;
  bytes member_pub_key_signature = 2; // Signed by invitation_priv_key
  bytes member_device_pub_key = 3;
  bytes member_device_pub_key_signature = 4; // Signed by member_priv_key

  bytes inviter_member_pub_key = 5;
  bytes invitation_pub_key = 6;
  bytes invitation_pub_key_signature = 7; // Signed by inviter_member_priv_key
}

Each MemberEntryPayload is an entry of a sigchain. You can see the signature / proof chain in the comment above.

The MemberEntryEnvelope is made this way:

• Fill all field of MemberEntryPayload
• Sign the MemberEntryPayload using sender_device_pub_key
• Set the signature as MemberEntryEnvelope.member_payload_signature
• Derive a symmetric key using member_payload_signature and group shared secret
• Encrypt MemberEntryPayload and set it as MemberEntryEnvelope.encrypted_member_payload