[#2] Add methods sign and verify

Signed-off-by: ilyas585 <niyazov2023@gmail.com>

frostfs_api/cryptography/signer.py

@@ -0,0 +1,24 @@
+import ecdsa
+from hashlib import sha256, sha512
+
+
+class Signer:
+    def sign_rfc6979(self, private_key: bytes, message: bytes) -> bytes:
+        if len(private_key) == 0 or private_key is None:
+            raise ValueError(f"Incorrect private_key: {private_key}")
+
+        sk = ecdsa.SigningKey.from_string(private_key, curve=ecdsa.NIST256p, hashfunc=sha256)
+
+        signature = sk.sign_deterministic(message)
+
+        return signature
+
+    def sign(self, private_key: bytes, message: bytes) -> bytes:
+        if len(private_key) == 0 or private_key is None:
+            raise ValueError(f"Incorrect private key: {private_key}")
+
+        sk = ecdsa.SigningKey.from_string(private_key, curve=ecdsa.NIST256p, hashfunc=sha512)
+
+        signature = sk.sign(message)
+
+        return signature