Support `jvm_opts` on scala_toolchain

Add support for `jvm_opts`, equivalent to java_toolchain: https://docs.bazel.build/versions/master/be/java.html#java_toolchain.jvm_opts

Start with the scala_toolchain implementation and compare it with the java_toolchain.jvm_opts behavior described in the linked Bazel documentation. Trace how toolchain options are passed to the JVM, then verify that scala_toolchain accepts and applies equivalent jvm_opts behavior.

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