[#310] *: Implement string converters for enumerations

Implement `String` / `FromString` method pair in all levels of enum
definitions. From now `String()` returns canonical protojson-compatible
values.

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

v2/session/string.go

@@ -0,0 +1,47 @@
+package session
+
+import (
+	session "github.com/nspcc-dev/neofs-api-go/v2/session/grpc"
+)
+
+// String returns string representation of ObjectSessionVerb.
+func (x ObjectSessionVerb) String() string {
+	return ObjectSessionVerbToGRPCField(x).String()
+}
+
+// FromString parses ObjectSessionVerb from a string representation.
+// It is a reverse action to String().
+//
+// Returns true if s was parsed successfully.
+func (x *ObjectSessionVerb) FromString(s string) bool {
+	var g session.ObjectSessionContext_Verb
+
+	ok := g.FromString(s)
+
+	if ok {
+		*x = ObjectSessionVerbFromGRPCField(g)
+	}
+
+	return ok
+}
+
+// String returns string representation of ContainerSessionVerb.
+func (x ContainerSessionVerb) String() string {
+	return ContainerSessionVerbToGRPCField(x).String()
+}
+
+// FromString parses ContainerSessionVerb from a string representation.
+// It is a reverse action to String().
+//
+// Returns true if s was parsed successfully.
+func (x *ContainerSessionVerb) FromString(s string) bool {
+	var g session.ContainerSessionContext_Verb
+
+	ok := g.FromString(s)
+
+	if ok {
+		*x = ContainerSessionVerbFromGRPCField(g)
+	}
+
+	return ok
+}