feat: Adding Plex Webhooks external plugin (influxdata#8898)

EXTERNAL_PLUGINS.md

@@ -9,6 +9,7 @@ Pull requests welcome.
 - [awsalarms](https://github.com/vipinvkmenon/awsalarms) - Simple plugin to gather/monitor alarms generated  in AWS.
 - [octoprint](https://github.com/BattleBas/octoprint-telegraf-plugin) - Gather 3d print information from the octoprint API.
 - [open-hardware-monitor](https://github.com/marianob85/open_hardware_monitor-telegraf-plugin) - Gather sensors data provided by [Open Hardware Monitor](http://openhardwaremonitor.org)
+- [plex](https://github.com/russorat/telegraf-webhooks-plex) - Listens for events from Plex Media Server [Webhooks](https://support.plex.tv/articles/115002267687-webhooks/).
 - [rand](https://github.com/ssoroka/rand) - Generate random numbers
 - [SMCIPMITool](https://github.com/jhpope/smc_ipmi) - Python script to parse the output of [SMCIPMITool](https://www.supermicro.com/en/solutions/management-software/ipmi-utilities) into [InfluxDB line protocol](https://docs.influxdata.com/influxdb/latest/reference/syntax/line-protocol/). 
 - [systemd-timings](https://github.com/pdmorrow/telegraf-execd-systemd-timings) - Gather systemd boot and unit timestamp metrics.