crypto: `impl signature::KeypairRef` for Signers

The various signers defined in `radicle-crypto` all allow borrowing
the corresponding public key a.k.a. "verifying key". This is captured
by implementing `signature::KeypairRef`, so implement it in addition
to `signature::Signer`.

crates/radicle-crypto/src/ssh/agent.rs

@@ -72,6 +72,16 @@ impl signature::Signer<ExtendedSignature> for AgentSigner {
     }
 }
 
+impl AsRef<PublicKey> for AgentSigner {
+    fn as_ref(&self) -> &PublicKey {
+        &self.public
+    }
+}
+
+impl signature::KeypairRef for AgentSigner {
+    type VerifyingKey = PublicKey;
+}
+
 impl AgentSigner {
     pub fn new(agent: Agent, public: PublicKey) -> Self {
         let agent = RefCell::new(agent);

crates/radicle-crypto/src/ssh/keystore.rs

@@ -265,6 +265,16 @@ impl signature::Signer<ExtendedSignature> for MemorySigner {
     }
 }
 
+impl AsRef<PublicKey> for MemorySigner {
+    fn as_ref(&self) -> &PublicKey {
+        &self.public
+    }
+}
+
+impl signature::KeypairRef for MemorySigner {
+    type VerifyingKey = PublicKey;
+}
+
 impl Signer for MemorySigner {
     fn public_key(&self) -> &PublicKey {
         &self.public

crates/radicle-crypto/src/test/signer.rs

@@ -23,6 +23,16 @@ impl signature::Signer<Signature> for MockSigner {
     }
 }
 
+impl AsRef<PublicKey> for MockSigner {
+    fn as_ref(&self) -> &PublicKey {
+        &self.pk
+    }
+}
+
+impl signature::KeypairRef for MockSigner {
+    type VerifyingKey = PublicKey;
+}
+
 impl MockSigner {
     pub fn new(rng: &mut fastrand::Rng) -> Self {
         let mut seed: [u8; 32] = [0; 32];