Implement new request signing mechanism

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

util/signature/options.go

@@ -0,0 +1,26 @@
+package signature
+
+import (
+	"crypto/ecdsa"
+
+	crypto "github.com/nspcc-dev/neofs-crypto"
+)
+
+type cfg struct {
+	signFunc   func(key *ecdsa.PrivateKey, msg []byte) ([]byte, error)
+	verifyFunc func(key *ecdsa.PublicKey, msg []byte, sig []byte) error
+}
+
+func defaultCfg() *cfg {
+	return &cfg{
+		signFunc:   crypto.Sign,
+		verifyFunc: crypto.Verify,
+	}
+}
+
+func SignWithRFC6979() SignOption {
+	return func(c *cfg) {
+		c.signFunc = crypto.SignRFC6979
+		c.verifyFunc = crypto.VerifyRFC6979
+	}
+}