Decodes DMs from mostro

[Catrya]

Communication between users and Mostro

https://docs.rs/mostro-core/latest/mostro_core/message/struct.MessageKind.html

[chebizarro]

PR #31 adds support for NIP-44 gift wraps and NIP-59 encrypted DMs. The NIP-44 implementation was bundled into a separate library - https://github.com/chebizarro/dart-nip44 which can be transferred to the mostro project if desired.
The mostro message kind has been implemented with a class extension to the regular NostrEvent to facilitate handling of DMs and open orders.
Is there anything outstanding or that requires clarification before closing this issue?

[Catrya]

completed in #31