Tidy up code

[zoidyzoidzoid]

We could structure the repo a little bit differently.

Maybe following something like this https://github.com/golang-standards/project-layout

So doing something like

cmd/server/main.go
pkg/handler.go

[zoidyzoidzoid]

For how simple this service is, it's probably overkill and more harm than its worth.

[catouc]

I think I took a stab at this with #14 🙃

I might do a bit more restructuring on the file layout the next few days, maybe move most of the core logic into it's own little package to separate the CLI config bits from the server logic.

[catouc]

https://github.com/Deichindianer/gitlab-honeycomb-buildevents-webhooks-sink/tree/deichindianer/restructure

I didn't create a PR for this since it's gotten a bit huge and looking at the diff won't actually be useful I think 🤔
It's created on top of #14 so it's fine to merge in order.

But I wanted to stop and see if that restructuring makes sense and should be upstreamed. I currently need to maintain a fork of the webhook lib since the upstream version has issues with BuildEvents parsing loosing data that we need, see: go-playground/webhooks#146

Oh and the interface to users should be entirely the same, the only thing I did not really get around to was to re-implement testing for some bits that I moved inline somewhere.

[catouc]

I think this can be closed after #15 was merged?