Add startup callback to settings for httpbeast

[ThomasTJdev]

Hi Dom,

A follow-up PR from the httpbeast PR dom96/httpbeast#89 :)

• This PR implements the startup proc in Jester' settings, which then is passed on to httpbeast. The startup proc will run in each httpbeast-thread at initialization.
• As is, this PR only implements the startup proc to be used, when jester is running with httpbeast.
• The concept for "only allowing" with httpbeast follows the previous numThreads implementation.

Would this be a possible PR? Or are there any changes you would like to see?

[dom96]

Weirdly (and this is unrelated to this PR) it seems @iffy's github action is installing the stable Nim, rather than the one defined by nimversion.

[iffy]

install-nim is expecting a variable named version not nimversion

Warning: Unexpected input(s) 'nimversion', valid inputs are ['version']

[iffy]

Change

jester/.github/workflows/main.yml

Line 29 in 185c610

nimversion: ${{ matrix.nimversion }}

to

          version: ${{ matrix.nimversion }}