Allow forcing SliderServer POST request using notebook metadata

[disberd]

Add the possibility of forcing Pluto to always use POST requests when requesting updates from a SliderServer.

Done by using a custom field in the notebook metadata.

This is mostly useful to avoid caching of the reply in cases where one has results that depend on random values and prefers to always get new random results for the same bond variables.

It is also useful to have SliderServer work better when one uses @bind calls generated inside custom made macros that are not correctly parsed by PlutoSliderServer to generate the bond graph

The easiest way to disable caching by enabling this feature is opening your notebook file (on the server) and adding this line at the beginning of the notebook

#> sliderserver_force_post = true

So that your notebook file starts like this (for example):

### A Pluto.jl notebook ###
# v0.19.26

#> sliderserver_force_post = true

[github-actions]

Try this Pull Request!

Open Julia and type:

julia> import Pkg
julia> Pkg.activate(temp=true)
julia> Pkg.add(url="https://github.com/disberd/Pluto.jl", rev="patch-5")
julia> using Pluto