MM-44953: Refactor EnsureBot to be used from plugin.API (#20521)

The mattermost-plugin-api code also needs to use this.
Hence we make it available from plugin.API.

https://mattermost.atlassian.net/browse/MM-44953

```release-note
NONE
```

app/plugin_api.go

@@ -981,6 +981,10 @@ func (api *PluginAPI) PermanentDeleteBot(userID string) *model.AppError {
 	return api.app.PermanentDeleteBot(userID)
 }
 
+func (api *PluginAPI) EnsureBotUser(bot *model.Bot) (string, error) {
+	return api.app.EnsureBot(api.ctx, api.id, bot)
+}
+
 func (api *PluginAPI) PublishUserTyping(userID, channelID, parentId string) *model.AppError {
 	return api.app.PublishUserTyping(userID, channelID, parentId)
 }