FR: Exposing a list of maven deps

Right now consuming the `gen_fhir_definitions_and_protos` rule is difficult for a project already using java/kotlin + maven deps as the [maven install](https://github.com/google/fhir/blob/master/bazel/workspace.bzl#L8) is unnamed and there is no exposing of the deps via a list of something of the like (which can be consumed via the project's own `maven_install`). This makes it only possible to manually copy the deps by hand, which is obviously error prone.

It would be really helpful to either expose the maven deps list (for example [how grpc-java does it](https://github.com/grpc/grpc-java/blob/master/repositories.bzl#L12) ), or maybe offer the ability to provide a [custom maven install name](https://github.com/bazelbuild/rules_jvm_external#custom-location-for-maven_installjson).

Apologies if there is a way to do this I'm missing. Thanks for a great library!