Enable auto-inference for Gradle and Maven codebases

[olafurpg]

We currently only support inference for JVM package repos like /jdk and /maven/.... We should consider infering index jobs for regular Java repos with Gradle and Maven builds.

The reason this is complicated to enable on sourcegraph.com is that many Gradle and Maven builds will fail to run scip-java index due to various issues (missing system dependencies, unsupported Gradle/Maven versions, ...). It's still possible to write custom inference logic with our new Lua support, which is likely to remain the recommended approach to auto-index Java repositories. Please contact Sourcegraph support if you need assistance to configure auto-indexing with Java repositories.