Feature request: support validating against past versions

[ZacSweers]

Currently, when new APIs are introduced, developers must manually review the changes and ensure they are binary compatible (because it's just a regenerated .api file). This is an error-prone process and we were burned by this in KotlinPoet recently where we accidentally missed a breaking API change amongst a number of other (expected) changes. One helpful feature of the japicmp gradle plugin is the ability to point to a baseline artifact to validate against. It would be great if this project could offer something similar, either via pointing at artifacts (ideally) or other ways.