Support worker cancellation

[jadenPete]

This issue is part of a broader proposal to merge some of the features in lucidsoftware/rules_scala into this ruleset.

The tracking issue for this effort

The relevant proposal section.

From the documentation on worker cancellation:

Workers can optionally allow work requests to be cancelled before they finish. This is particularly useful in connection with dynamic execution, where local execution can regularly be interrupted by a faster remote execution. To allow cancellation, add supports-worker-cancellation: 1 to the execution-requirements field (see below) and set the --experimental_worker_cancellation flag.

Worker cancellation will be particularly helpful when using the ruleset with remote execution.