Adds a watch flag to the get command

The new flag fetch and display the request ressource and then continue
watching the ressource until timeout or cancellation.

A single ressource/ressource type is supported.

Signed-off-by: Soule BA <soule@weave.works>

cmd/flux/get_source_all.go

@@ -34,6 +34,11 @@ var getSourceAllCmd = &cobra.Command{
   # List all sources in all namespaces
   flux get sources all --all-namespaces`,
 	RunE: func(cmd *cobra.Command, args []string) error {
+		err := validateWatchOption(cmd, "all")
+		if err != nil {
+			return err
+		}
+
 		var allSourceCmd = []getCommand{
 			{
 				apiType: bucketType,