refactor: autoloading via zeitwerk

[YOU54F]

fixes #633

Lots of files touched, but mainly the removal of require "pact_broker/.*"

Main loading logic for Zeitwerk is in our gems entrypoint ./lib/pact_broker.rb which follows the pattern

1. Load zeitwerk germ
2. We instruct zeitwerk not to warn us about files that are unmanaged by Zeitwerk, these are because we cannot rename or rework the files to work within convention, we just need to manually load these as before
3. We use Zeitwerk:Loader.for_gem which users the file list in the gemspec to generate a load list of files
4. We explicitly ignore the files Zeitwerk cannot load, these were identified by commenting out step 2
5. We call loader.setup to autoload all not ignored files
6. We manually require pact_broker/webmachine which loads webmachine, and our ignored monkey patches

The rest of the changes are just renaming files/classes/modules to match Zeitwerk conventions, and removal of manual imports which are no longer required, as that is the big benefit of using an autoloader.

[YOU54F]

Yoooo @bethesque!

I had a bit of a go at this, after getting familiarised with Zeitwerk in the pact-ruby v2 rewrite.

Thought you might fancy a goosey gander.

Wasn't too bad in the end actually, probably in part ot the work you have previously done in #661

[YOU54F]

Hey team!

We may need some coordination, and/or testing with PactFlow prior to merging this in, in case there are changes required to consume, or if we need to plan it in with some engineering time in order to be able to apply this.

[mefellows]

@impurist @pahnin FYI