[#74] apemanager: Generate protobufs for apemanager service

* Generate protobufs.
* Make marshallers, unmarshallers, json-encoders etc.
* Create message encoding/decoding unit-tests.

Signed-off-by: Airat Arifullin <a.arifullin@yadro.com>

apemanager/string.go

@@ -0,0 +1,18 @@
+package apemanager
+
+import (
+	apemanager_grpc "git.frostfs.info/TrueCloudLab/frostfs-api-go/v2/apemanager/grpc"
+)
+
+func (tt TargetType) String() string {
+	return TargetTypeToGRPCField(tt).String()
+}
+
+func (tt *TargetType) FromString(s string) bool {
+	i, ok := apemanager_grpc.TargetType_value[s]
+	if ok {
+		*tt = TargetType(i)
+	}
+
+	return ok
+}