Lua: add function pandoc.with_state

[tarleb]

The function allows to run a callback with a modified pandoc state. This
provides the ability to temporarily modify the resource path, the user
data directory, and the HTTP request headers.

Closes: #10859

[kainctl]

Hey, this is great! Thanks!
One question though, why silently ignore errors?
I tried this feature and it didn't seem to work at first. Only later did I discover that I typed resource_paths instead of resource_path.

[tarleb]

Honest answer: because checking for invalid options is more work and didn't strike me as necessary at first. We can add it though, please raise an issue so we won't forget.