Update publish plugin to expose an API for using pre-existing pom file

### Expected Behavior

When wanting to publish an already built gradle plugin artifact in the Plugin portal, you want to use the existing pom file. See https://github.com/gradle/plugin-portal-requests/issues/9#issuecomment-566224151.

The `pluginBundle` DSL should provide a way to use the existing pom file (either as a supplied file or as the pom resource in the artifact jar) rather than always generating a new pom file.

### Current Behavior

The publish plugin always generates a new pom file which will not contain necessary information already in the existing pom file.

### Context

See https://github.com/gradle/plugin-portal-requests/issues/9.