MM-10703 Adding blank request context to plugin hooks for future use. (#9043)

* Adding blank request context to plugin hooks for future use.

* Rename RequestContext to Context

* Adding context to ServeHTTP and ExecuteCommand

* Fixing import cycle in test.

app/plugin_commands.go

@@ -9,6 +9,7 @@ import (
 	"strings"
 
 	"github.com/mattermost/mattermost-server/model"
+	"github.com/mattermost/mattermost-server/plugin"
 )
 
 type PluginCommand struct {
@@ -104,7 +105,7 @@ func (a *App) ExecutePluginCommand(args *model.CommandArgs) (*model.Command, *mo
 			if err != nil {
 				return pc.Command, nil, model.NewAppError("ExecutePluginCommand", "model.plugin_command.error.app_error", nil, "err="+err.Error(), http.StatusInternalServerError)
 			}
-			response, appErr := pluginHooks.ExecuteCommand(args)
+			response, appErr := pluginHooks.ExecuteCommand(plugin.NewBlankContext(), args)
 			return pc.Command, response, appErr
 		}
 	}