Add startup callback to settings for httpbeast + fix CI

[ThomasTJdev]

Notes:

• Previous PR Add startup callback to settings for httpbeast #318 was closed due to deleted branch in fork.. mistake.
• This PR also includes the CI fix specified by iffy and available as independent PR Update main.yml to compile with specified nim version #319

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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?

[ThomasTJdev]

CI will fail until httpbeast has a new release tag.