Add ability to manually send analytics POST request from the server

[obulat]

Problem

vue-plausible only works on the client side. The search_time_event needs to be send both from the client and from the server.

Description

The ideal solution would be to create a module that would also expose plausible tracker on the server. However, this is too complex and time-consuming, and considering that it would only work with Nuxt 2, which is EOL soon, it is not feasible.
A hot-fix solution we can use is to send the request from the server, filling in all of the required properties manually.
Here are the properties that need to be added:

https://github.com/plausible/plausible-tracker/blob/c0b87d997d839938c23023d35bac0d6683635bbc/src/lib/request.ts#L61-L69

Blockers

This issue is blocked on #3777