v2: Define API service interfaces

Define interfaces that reflect service (Accounting, Container, Session)
declarations in the API.

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

v2/session/service.go

@@ -0,0 +1,25 @@
+package session
+
+import (
+	"context"
+)
+
+type Service interface {
+	Create(context.Context, *CreateRequest) (*CreateResponse, error)
+}
+
+type CreateRequest struct {
+	body *CreateRequestBody
+
+	metaHeader *RequestMetaHeader
+
+	verifyHeader *RequestVerificationHeader
+}
+
+type CreateResponse struct {
+	body *CreateResponseBody
+
+	metaHeader *ResponseMetaHeader
+
+	verifyHeader *ResponseVerificationHeader
+}