Simplify and Automate Release Workflows with a New "bazel release" Command

[sluongng]

Disclaimer: rough personal idea of mine. Many rough edges.

Today, repositories that use Bazel often have difficulty determining "when" to release an artifact.
Many of these rely on bazel {,c}query to recreate Bazel's internal dependency graph and combine it with the change diff to determine the list of deployable targets.

I think to make this simpler to end users, we can consider adding a new Bazel subcommand bazel release <targets>. Similar to bazel test <targets>, this new subcommand should build all the <targets> provided by the user and then run executable binaries that are marked as "is_release".

To help with that, we can extend rule() to add a new boolean attribute called is_release. Alternatively, we can also consider deprecating the test attribute in favor of something like executable_type, which takes one of none, test, release values. "release" rules must return ExecutionInfo to define how the binary should be executed at the "release phase"

The release actions can be called ReleaseRunner, similar to the current TestRunner actions. ReleaseRunner actions are special in that they should be cached solely by their release binary digest. The execution environment (i.e. environment variables) should not be part of the cache key so secrets/credentials could be provided at runtime.

Additionally, the ExecutionInfo provider could be used by the rule author to customize:

• Sandboxing options (using no-sandboxing or a persistent worker might allow the binary to access secrets/credentials value. Persistent worker might help control the concurrency as well.). Worth noting that this is not mandatory since you can mount the credential files into Bazel's sandbox today.
• Networking options
• Caching enabled/disabled

This should also enable these release workflows to benefit from RBE where these release actions could be run parallelly on a remote compute environment and (hopefully) deduplicated by the RBE scheduler.

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Ps: I know that we have bazel mobile-install but that's not generalizable.