Add support for the Events API

[andrerfcsantos]

Recently Plausible introduced the Events API that allows custom events to be sent to Plausible. go-plausible should also support this endpoint.

Notes about this task

• Since this request requires a domain, this probably should be implemented as method on Site , which contains the domain in the id field
• This endpoint requires information to be sent in the body of the request. The method client.acquireRequest can be used for this, where formVals QueryArgs represents the body data.
• This endpoint also requires some extra headers compared to the other requests, as mentioned on the Plausible docs.

Tasks

• Implement the functionality for the new kind of request
• Write tests for the new functionality
• Update the documentation

[till]

@andrerfcsantos hey 👋🏼 — any thoughts on my PR?