Pagination in the Combined Status API

We're getting close to bringing the Combined Status API out of preview mode, and have just a couple of small changes to make before it's :sparkles:.

First, we're now paginating combined status API calls. The combined status state field will always take all statuses into account, but we'll now only return 100 embedded statuses at a time.

Second, we're adding a total_count field, mirroring the Search API. This count represents the number of contexts submitted for the given commit.

As always, we're interested in hearing your feedback.