Release the java runfiles library as a maven coordinate

When working on a project that doesn't build with bazel but needs to use the runfiles library (for example, when porting a project from gradle to bazel) there's no easy way to access the java runfiles library. [The Python world exports theirs as a wheel](https://pypi.org/project/bazel-runfiles/).

It would be helpful to expose the java runfiles library as a maven coordinate.

Bazel projects can either not use the coordinate or use a `maven.override` to point to the Bazel-provided version.