refactor internals, add initial gitlab support

[colemickens]

1. Refactored most of flakehub-push's internals:
   • There were a lot of places where we were bulk copying around values (such that we had to disable clippy lints), often times variables names differed on each side, leading to, for me, quite a bit of confusion
   • There's still at least two places where we branch off for GitLab/GitHub. This could potentially be improved in the future, but the way that we fill from CLI, backfill from env, fill from API, etc,... it wasn't as trivial as I'd hoped.
   • There's an attempt to prevent confusion between using the initial cli-provided values, and the new PushContext values that are the result of the various processing/backfilling we do.
2. Added initial, albeit limited GitLab API support
   • Question: how to handle commit_count given that I don't think GitLab gives us a way to get it. I think there's a small additional logic change I can make to at least make commit_count optional for GitLab.

[cole-h]

Not expecting this to be done now, but maybe we should leave a note about employing some library that allows us to detect the SPDX at least from a LICENSE{,.md} file.