Cloud migrations: create snapshot files (#89693)

* Cloud migrations: create snapshot and store it on disk

* fix merge conflicts

* implement StartSnapshot for gms client

* pass snapshot directory as argument to snapshot builder

* ensure snapshot folder is set

* make swagger-gen

* remove Test_ExecuteAsyncWorkflow

* pass signed in user to buildSnapshot method / use github.com/grafana/grafana-cloud-migration-snapshot to create snapshot files

* fix FakeServiceImpl.CreateSnapshot

* remove new line

pkg/services/cloudmigration/model.go

@@ -195,8 +195,12 @@ type CreateSessionResponse struct {
 	SnapshotUid string
 }
 
-type InitializeSnapshotResponse struct {
-	EncryptionKey  string
-	UploadURL      string
-	GMSSnapshotUID string
+type StartSnapshotResponse struct {
+	SnapshotID           string            `json:"snapshotID"`
+	MaxItemsPerPartition uint32            `json:"maxItemsPerPartition"`
+	Algo                 string            `json:"algo"`
+	UploadURL            string            `json:"uploadURL"`
+	PresignedURLFormData map[string]string `json:"presignedURLFormData"`
+	EncryptionKey        string            `json:"encryptionKey"`
+	Nonce                string            `json:"nonce"`
 }