Allow custom stub template for java_binary

### Description of the feature request:

We would like to use our own java stub template to replace java_stub_template.txt. Our use case uses a lot of custom system properties that can be generated from rule dependencies. However Bazel's java stub template is fixed. It would be nice to allow custom java rule template through something like java_toolchain

Start at java_stub_template.txt and the java_toolchain entry point to understand how java_binary currently selects its stub template. Define how a user-supplied template would be passed and validated, while preserving the default; done means java_binary can use a custom template for dependency-derived system properties.

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