EncryptedDirectMessage class; simplify Event

[kdmukai]

This one probably should have been separated out into 3-4 small PRs. I can pull these apart if need be.

EncryptedDirectMessage class

Derived from the parent Event class to provide a more human-friendly way to construct a DM:

from nostr.event import EncryptedDirectMessage

dm = EncryptedDirectMessage(
  recipient_pubkey=recipient_pubkey,
  cleartext_content="Secret message!"
)
private_key.sign_event(dm)
relay_manager.publish_event(dm)

• The recipient's 'p' reference tag is automatically inserted for us.
• 'cleartext_contentprovides a clean way to differentiate the cleartext content from the ultimate encryptedcontent`.
• Trying to instantiate a DM by specifying content throws an exception.
• PrivateKey.sign() encrypts the message for us and populates the content field.
• Explicit PrivateKey.encrypt_dm() convenience method added while preserving the more general encrypt_message() in case that finds other uses(?).
• The DMs are still Event types and so can be passed straight on to the RelayManager.
• Relevant tests added.
• Example added to README.

Event.__init__ boilerplate removed via dataclass

Boilerplate bad. DRY good.

Simplified Event instantiation

We're going to sign an Event with our PrivateKey anyway so there's no need to populate the Event.public_key field on instantiation.

event = Event("Hello Nostr")
private_key.sign_event(event)
relay_manager.publish_event(event)

This same simplification is also demonstrated in the EncryptedDirectMessage example above.

README examples updated accordingly.

Event explicit 'e' and 'p' tag support

Similar to the discussion in #12:

• Add explicit add_event_ref(event_id) to generate 'e' tags
• Add explicit add_pubkey_ref(pubkey) to generate 'p' tags
• README example added for replying to a note.

[callebtc]

ACK

Type annotations good

[jeffthibault]

Thanks @kdmukai! Did a first pass and added a few comments and questions.

[kdmukai]

@jeffthibault great notes! All my updates are now committed.

• Simpler Event instantiation w/content string w/out kwarg.
• tags attr default value via factory.
• id changed to a compute-on-the-fly property; eliminates all the redundant compute_id() calls.
• Simpler DM instantiation (same as Event); simply copies the content field over to cleartext_content and no longer throws Exception.
• Asking for a DM's id raises an Exception if its message hasn't been encrypted yet.
• Adds/updates relevant Event tests.
• Restores key.mine_vanity_key.

[jeffthibault]

Thanks @kdmukai. This looks great!

Tested ACK.