[#2] Add methods sign and verify

tests/client/test_sign_verify.py

@@ -0,0 +1,29 @@
+import ecdsa
+import pytest
+
+from frostfs_api.client.signer import Signer
+from frostfs_api.client.verifier import Verifier
+from frostfs_api.crypto.key_extension import KeyExtension
+from tests.helpers.resources import WIF
+
+
+@pytest.mark.crypto
+class TestSignAndVerify:
+
+    @pytest.mark.parametrize("message", [b"Hello Frostfs API", b""])
+    def test_sign_verify_success(
+            self,
+            client_signer: Signer,
+            client_verifier: Verifier,
+            client_key_extension: KeyExtension,
+            message: bytes
+    ):
+        private_key = client_key_extension.get_private_key_from_wif(WIF)
+        public_key = client_key_extension.get_public_key(private_key)
+
+        signature = client_signer.sign_rfc6979(private_key, message)
+        assert client_verifier.verify_rfc6979(public_key, message, signature) is True
+
+    def test_verify_rfc6979_empty_key(self, client_verifier: Verifier):
+        with pytest.raises(ValueError, match="Incorrect public key"):
+            client_verifier.verify_rfc6979(b'', b"Test message", b"signature")