Add buildscript helpers to `ethcontract-generate`

[nlordell]

It would be beneficial for newer projects to get up and running with less friction if some of the helpers we use for generating contract bindings in build scripts were moved to a shared location. They could facilitate patterns like we have in gp-v2-services including things like supporting "address files" that we use for E2E testing.

[taminomara]

Idea from #512: Implement ArtifactBuilder to build all/some contracts from an artifact.