Feature request: --show-rulekey

### Description of the problem / feature request:

Buck has the concept of [rule keys](https://buckbuild.com/concept/rule_keys.html), which we can obtain by running `buck targets --show-rulekey //...`. We'd love to see similar feature in Bazel.

### Feature requests: what underlying problem are you trying to solve with this feature?

We need to see what targets are changed, thus need to rebuilt and test, from one revision to another. In order to [keep master green at scale](https://dl.acm.org/citation.cfm?id=3303970), we need to build and test diffs in parallel. Getting the rule key for each target help us decide what diffs are independent and safe to build and test in parallel.

Start by reading the linked rule-key concept and comparing Bazel's target behavior with Buck's `buck targets --show-rulekey //...` command. Define how Bazel should expose a rule key for each target and verify that the output supports identifying changed targets for parallel build and test decisions.

Written by the indexing model from the issue text.