feat: support creating version with branch

[bethesque]

Adds a branch property to the version resource. This is the first step towards providing first class "branch" and "deployment environments" as a more understandable alternative to "tags".

Next step is to support the branch property for both consumer and provider in the "pacts for verification" API.

[bethesque]

Don't try and read this all - this PR contains a massive clean up of the terrible code that used to be there for the "pacts for verification" finding. It also splits out some massive files that Code Climate was rightly complaining about.

The things worth knowing:

• There's now an endpoint to create a version with a branch and a build URL

   PUT http://broker/pacticipants/Bar/versions/1e70030c6579915e5ff56b107a0fd25cf5df7464
    {
      "branch": "main",
      "buildUrl": "http://ci.mydomain/my-job/1"
    }
  

• Publishing pacts and verifications should now support a "branch" property, and the client libraries should all make the above call before creating the tags (I haven't made an "all in one" call for the version and the tags yet).
• When calling the 'pacts for verification' API
  • the consumer version selectors now support branch and fallbackBranch
  • there is an extra providerVersionBranch property
  • If a provider version branch AND tags are specified, the branch is used in preference to the tags when calculating the "work in progress" pacts
  • If only provider version tags are specified, it works the way it currently does
  • When calculating which pacts are WIP, both the "latest for each tag" and "latest for branch" are considered.

[bethesque]

Oh, there's also a pb:pacticipant-version relation in the index now which should be used:

1. to detect whether or not the broker version supports creating a pacticipant version by PUT, and by extension, whether or not the branch property is supported
2. to actually create the pacticipant version