feat: rewrite the nip-4e implementation (#1)

Make NIP-4e a core feature, not an optional feature.

Note:
- The UI is broken and needs to be updated in a separate PR.

Reviewed-on: #1

crates/state/src/identity.rs

@@ -0,0 +1,86 @@
+use nostr_sdk::prelude::*;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub enum RelayState {
+    #[default]
+    Initial,
+    NotSet,
+    Set,
+}
+
+impl RelayState {
+    pub fn is_initial(&self) -> bool {
+        matches!(self, RelayState::Initial)
+    }
+}
+
+/// Identity
+#[derive(Debug, Clone, Default)]
+pub struct Identity {
+    /// The public key of the account
+    pub public_key: Option<PublicKey>,
+
+    /// Status of the current user NIP-65 relays
+    relay_list: RelayState,
+
+    /// Status of the current user NIP-17 relays
+    messaging_relays: RelayState,
+}
+
+impl AsRef<Identity> for Identity {
+    fn as_ref(&self) -> &Identity {
+        self
+    }
+}
+
+impl Identity {
+    pub fn new() -> Self {
+        Self {
+            public_key: None,
+            relay_list: RelayState::default(),
+            messaging_relays: RelayState::default(),
+        }
+    }
+
+    /// Sets the state of the NIP-65 relays.
+    pub fn set_relay_list_state(&mut self, state: RelayState) {
+        self.relay_list = state;
+    }
+
+    /// Returns the state of the NIP-65 relays.
+    pub fn relay_list_state(&self) -> RelayState {
+        self.relay_list
+    }
+
+    /// Sets the state of the NIP-17 relays.
+    pub fn set_messaging_relays_state(&mut self, state: RelayState) {
+        self.messaging_relays = state;
+    }
+
+    /// Returns the state of the NIP-17 relays.
+    pub fn messaging_relays_state(&self) -> RelayState {
+        self.messaging_relays
+    }
+
+    /// Force getting the public key of the identity.
+    ///
+    /// Panics if the public key is not set.
+    pub fn public_key(&self) -> PublicKey {
+        self.public_key.unwrap()
+    }
+
+    /// Returns true if the identity has a public key.
+    pub fn has_public_key(&self) -> bool {
+        self.public_key.is_some()
+    }
+
+    /// Sets the public key of the identity.
+    pub fn set_public_key(&mut self, public_key: PublicKey) {
+        self.public_key = Some(public_key);
+    }
+
+    /// Unsets the public key of the identity.
+    pub fn unset_public_key(&mut self) {
+        self.public_key = None;
+    }
+}