Add startup callback to settings for httpbeast + fix CI

[ThomasTJdev]

Notes:

• Previous PR https://github.com/dom96/jester/pull/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 https://github.com/dom96/jester/pull/319

---

Hi Dom,

A follow-up PR from the httpbeast PR https://github.com/dom96/httpbeast/pull/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.