TRIGGER CI: Reimplement the jvm tool registration mechanism via the options system.

Instead of reaching in to config directly, the tool registration simply
registers an appropriately-named option, and then registers the fact
that this option is in fact a jvm tool.

The option value is a list of specs. We set sensible defaults in the code,
which are specs in a root-level BUILD.tools file.

There's one slight quirk, which is that we must the register function into
our calls to register_jvm_tool(). This is because the JvmToolTaskMixin can't
have its own register_options() method (because it comes after TaskBase
in the mro, and chained super-calls to register_options() stop at TaskBase).