plugin register with artifact stubs VAULT-32686 (#29113)

* add plugin catalog's entValidate() and setInternal() oss stubs 
* create plugin register command constructor oss stub
* create EntPluginRunner oss stub
* add validateSHA256() oss stub to validate plugin catalog update input

command/commands.go

@@ -551,9 +551,7 @@ func initCommands(ui, serverCmdUi cli.Ui, runOpts *RunOptions) map[string]cli.Co
 			}, nil
 		},
 		"plugin register": func() (cli.Command, error) {
-			return &PluginRegisterCommand{
-				BaseCommand: getBaseCommand(),
-			}, nil
+			return NewPluginRegisterCommand(getBaseCommand()), nil
 		},
 		"plugin reload": func() (cli.Command, error) {
 			return &PluginReloadCommand{