[#337] client: Add option to specify callback of response information

There is a need to analyze some response information in API client.

Define `ResponseMetaInfo` structure of response information. Add
`WithResponseInfoHandler` client option which allows to set the response
info callback. The callback is called right after any response is received.

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

pkg/client/response.go

@@ -0,0 +1,37 @@
+package client
+
+import (
+	"github.com/nspcc-dev/neofs-api-go/v2/session"
+)
+
+// ResponseMetaInfo groups meta information about any NeoFS API response.
+type ResponseMetaInfo struct {
+	key []byte
+}
+
+// ResponderKey returns responder's public key in a binary format.
+//
+// Result must not be mutated.
+func (x ResponseMetaInfo) ResponderKey() []byte {
+	return x.key
+}
+
+// WithResponseInfoHandler allows to specify handler of response meta information for the all Client operations.
+// The handler is called right after the response is received. Client returns handler's error immediately.
+func WithResponseInfoHandler(f func(ResponseMetaInfo) error) Option {
+	return func(opts *clientOptions) {
+		opts.cbRespInfo = f
+	}
+}
+
+func (c *clientImpl) handleResponseInfoV2(opts *callOptions, resp interface {
+	GetVerificationHeader() *session.ResponseVerificationHeader
+}) error {
+	if c.opts.cbRespInfo == nil {
+		return nil
+	}
+
+	return c.opts.cbRespInfo(ResponseMetaInfo{
+		key: resp.GetVerificationHeader().GetBodySignature().GetKey(),
+	})
+}