bearer: Provide method to decode signed fields

It is in demand by apps exchanging unsigned tokens.

Signed-off-by: Leonard Lyubich <leonard@morphbits.io>

bearer/bearer.go

@@ -270,11 +270,24 @@ func (b *Token) Sign(signer neofscrypto.Signer) error {
 
 // SignedData returns actual payload to sign.
 //
-// See also [Token.Sign].
+// See also [Token.Sign], [Token.UnmarshalSignedData].
 func (b *Token) SignedData() []byte {
 	return b.signedData()
 }
 
+// UnmarshalSignedData is a reverse op to [Token.SignedData].
+func (b *Token) UnmarshalSignedData(data []byte) error {
+	var body acl.BearerTokenBody
+	err := body.Unmarshal(data)
+	if err != nil {
+		return fmt.Errorf("decode body: %w", err)
+	}
+
+	var tok acl.BearerToken
+	tok.SetBody(&body)
+	return b.readFromV2(tok, false)
+}
+
 // VerifySignature checks if Token signature is presented and valid.
 //
 // Zero Token fails the check.

bearer/bearer_test.go

@@ -289,6 +289,11 @@ func TestToken_SignedData(t *testing.T) {
 
 	require.False(t, val.VerifySignature())
 
+	signedData := val.SignedData()
+	var dec bearer.Token
+	require.NoError(t, dec.UnmarshalSignedData(signedData))
+	require.Equal(t, val, dec)
+
 	signer := test.RandomSignerRFC6979(t)
 	val = bearertest.Token(t)