Allow support for manual sharding for go_test rule

The go_test rule runs a round robin strategy for assigning tests to shards. I want to implement a custom sharding strategy in my application but there is no way around the default one enforced by go_test.

Start by tracing the go_test rule and how it assigns tests to shards, then identify where the current round-robin strategy is enforced. Define how an application can provide a custom sharding strategy and verify that go_test uses it instead of the default behavior.

Written by the indexing model from the issue text.