Add support for nullaway-bom

[hazendaz]

No bug or issue, but given nullaway is a number of different modules, it would be very useful to have a nullaway-bom produced. I'm sure many are in the same boat and its a chore to load the dependencies one by one as we are separated from maven central. Many projects have started going this direction which then makes it incredibly easy to pull one artifact and get all of them at once. The bom would just be a pom file that contains all the artifacts of the project.

I'm not a gradle user, but there is a gradle example of such a thing here https://github.com/mockito/mockito/tree/main/mockito-bom.