Add API to get "BITBUCKET_BUILD_NUMBER"

Currently, there's no way for the user to get the build number via the API. Having this will help user as the following use case:

I use the Build Number as an internal version number. It's is used for the app's version code and also displayed as part of the version name on certain builds. My intent is to fetch the latest Build Number from the repository instead of manually updating.

The exercise is mostly a POC and not critical should exposing these variables require a lot of effort; however, given that we're able to update the Build Number, I feel that access to read it would make this more complete.