Implement signature interface in request headers

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

service/v2/signature.go

@@ -0,0 +1,17 @@
+package v2
+
+func (m *RequestMetaHeader) ReadSignedData(buf []byte) ([]byte, error) {
+	return m.StableMarshal(buf)
+}
+
+func (m *RequestMetaHeader) SignedDataSize() int {
+	return m.StableSize()
+}
+
+func (m *RequestVerificationHeader) ReadSignedData(buf []byte) ([]byte, error) {
+	return m.StableMarshal(buf)
+}
+
+func (m *RequestVerificationHeader) SignedDataSize() int {
+	return m.StableSize()
+}