feat: support multiple branches per version

[bethesque]

tl;dr support an application version being associated with multiple branches.

This keeps track of the branch head versions, so we won't have same performance issues we do with "latest for tag" (though I'm going to use the same design to fix the performance issues with tags too at some stage).

Adding branch support to the Pact clients

@uglyog to associate a version with a branch, it's pretty much the same as tagging:

    curl -XPUT http://broker/pacticipants/{pacticipant}/branches/{branch}/versions/{version} \
          -H "Content-Type: application/json" -d "{}"

Like the tagging, the resource will auto create the pacticipant, branch and version if they don't exist. Also like the tagging, it should be called before the pact is published so that the right data is set up on the version when the webhook gets kicked off.

The all-in-one contract publishing endpoint is already set up to support the branch property, but if you'd rather just add to the mess that is the existing pact publishing code for now and do a separate call, the HAL relation to create the branch version is on the pacticipant resource at pb:branch-version or the index atpb:pacticipant-branch-version. Please detect the presence of that relation to determine whether or not branches are supported and show an appropriate "not supported, please upgrade" message to the user. The Ruby client says "This version of the Pact Broker does not support version branches. Please update to version 2.58.0 or later." (I'm just guessing for now that will the the version it gets released in, it'll be near enough.)

I haven't created the "all in one" verification publishing endpoint yet, so for now, you'll have to use the branch version endpoint to set the branch for provider versions anyway.

[uglyog]

Lots of changes. Must be good.

[bethesque]

More changes must be more better right? (Not expecting anyone to care about the code, just getting you both across the features).