[Webhook] Implement new service plugin

[jpmens]

We want a webhook service. It should have configurable headers it can send on outgoing POST and of course a configurable/templateable payload.

I'm thinking along the lines of MQTT publish to mqttwarn to Webhook to trigger whatever.

[jusplainmike]

I just came accross this after updating my Discord service. I will see about creating a generic webhook template.

[amotl]

@jusplainmike: Apprise has a Notify_discord plugin and Apprise support has recently been added to mqttwarn through 6fb7b80 (Apprise plugin documentation), see also #400.

[jusplainmike]

Yes thank you, its been working well.