Add age plugin support

See https://github.com/str4d/rage/tree/main/age-plugin.

Example usage:

	RAGE_DIR=$PWD/../Rust/rage
	(cd $RAGE_DIR; cargo build --all)
	export PATH=$PATH:$RAGE_DIR/target/debug

	age-plugin-trezor -i "John Doe" | tee trezor.id
	R=$(grep recipient trezor.id | cut -f 3 -d ' ')

	date | tee msg.txt
	rage -er $R < msg.txt > enc.txt
	rage -di trezor.id < enc.txt

libagent/age/client.py

@@ -0,0 +1,48 @@
+"""Device abstraction layer for AGE operations."""
+
+import logging
+
+from cryptography.hazmat.primitives import hashes
+from cryptography.hazmat.primitives.kdf.hkdf import HKDF
+
+from ..device import interface
+
+log = logging.getLogger(__name__)
+
+
+def create_identity(user_id):
+    """Create AGE identity for hardware device."""
+    result = interface.Identity(identity_str='age://', curve_name="ed25519")
+    result.identity_dict['host'] = user_id
+    return result
+
+
+class Client:
+    """Sign messages and get public keys from a hardware device."""
+
+    def __init__(self, device):
+        """C-tor."""
+        self.device = device
+
+    def pubkey(self, identity, ecdh=False):
+        """Return public key as VerifyingKey object."""
+        with self.device:
+            pubkey = bytes(self.device.pubkey(ecdh=ecdh, identity=identity))
+            assert len(pubkey) == 32
+            return pubkey
+
+    def ecdh(self, identity, peer_pubkey):
+        """Derive shared secret using ECDH from peer public key."""
+        log.info('please confirm AGE decryption on %s for "%s"...',
+                 self.device, identity.to_string())
+        with self.device:
+            assert len(peer_pubkey) == 32
+            result, self_pubkey = self.device.ecdh_with_pubkey(
+                pubkey=(b"\x40" + peer_pubkey), identity=identity)
+            assert result[:1] == b"\x04"
+            hkdf = HKDF(
+                 algorithm=hashes.SHA256(),
+                 length=32,
+                 salt=((peer_pubkey + self_pubkey)),
+                 info=b"age-encryption.org/v1/X25519")
+            return hkdf.derive(result[1:])