feat: [POC] send webhooks when nodes are purged (i.e. Next on-demand revalidation support)

[jasonbahl]

This is a proof-of-concept for sending webhooks in response to purge actions tracked by WPGraphQL Smart Cache.

Why?

This can help facilitate remote sources reacting to changes in the CMS. Some common applications:

• NextJS On Demand Revalidation
• Sending payloads to logging services for debugging, etc
• Notification systems (i.e. Slack notifications, etc)

Why not use another webhook plugin?

WPGraphQL Smart Cache already handles a lot of the logic required to determine whether a node is public or private, etc and centralizes a lot of mechanisms. By piggy-packing on the existing functionality we can provide a lot of value with minimal code.

How it works:

• WPGraphQL Smart Cache tracks events (i.e. post updated, post deleted)
• WPGraphQL Smart Cache emits a purge event as do_action( 'graphql_purge', $key, $event, $host );
• When the graphql_purge event is called, a $webhook_configs array is iterated over to determine which endpoint(s) to send webhook(s) to
• Each endpoint configured is sent a webhook as a POST or GET request with a payload (either POST body or GET query params)
• The payload and destination can be configured / filtered to meet various needs, but the default payload at the moment looks like:

GET:

{
"url": "http://localhost:3000/api/revalidate/?key=dGVybToxMQ=&event=term_saved&hostname=uri-debugging.local/graphql&path=/tag/tag-1/",
"method": "GET",
"headers": [],
"body": null,
"blocking": false
}

POST

{
"url": "http://localhost:3000/api/revalidate/",
"method": "POST",
"headers": {
"Content-Type": "application/json"
},
"body": "{\"key\":\"dGVybToxMQ==\",\"event\":\"term_saved\",\"hostname\":\"uri-debugging.local\\/graphql\",\"path\":\"\\/tag\\/tag-1\\/\"}",
"blocking": false
}

[jasonbahl]

I converted this PR to a draft as it's a Proof of Concept more than production ready. If we want to work on this and make it production ready we'll need to scope it out and figure out what all is required for this.