Specify JVM Repository in WORKSPACE

Would be useful to prevent users from having to install a JVM when contributing -- and to use a standard JVM version (as opposed to what is installed on their box). The main bazel repo does something like this.

Start with WORKSPACE and compare the main Bazel repository's JVM repository setup, as suggested in the issue. Determine how rules_jvm_external can provide a standard JVM without requiring a local installation; done means contributors can use the declared JVM version through the repository configuration.

Written by the indexing model from the issue text.