Add support to provide raw classpath to `lsif-java.json`

[olafurpg]

Currently, the lsif-java.json build file only supports configuring the classpath through the dependencies key, which takes a list of Maven coordinates that lsif-java then resolves to a classpath. In some cases, it could be helpful for users to provide the raw compile classpath instead.

[olafurpg]

I started working on this here https://github.com/sourcegraph/lsif-java/compare/main...olafurpg:raw-classpath?expand=1 but will need to pick up again later. Not sure how to structure the docs yet.