Add test sharding support for rust_test

[brianduff]

Problem

rust_test targets don't support Bazel's native test sharding. When shard_count is set on a rust_test:

• Without --incompatible_check_sharding_support: all tests run N times (once per shard)
• With --incompatible_check_sharding_support: the test fails because Rust's libtest harness doesn't read TEST_SHARD_INDEX/TEST_TOTAL_SHARDS environment variables

Solution

This PR adds opt-in sharding support via a new experimental_enable_sharding attribute. When enabled, tests are executed via a wrapper script that:

1. Detects sharding via TEST_TOTAL_SHARDS environment variable
2. Touches TEST_SHARD_STATUS_FILE to advertise sharding support to Bazel
3. Enumerates tests using libtest's --list --format terse flag
4. Partitions tests by index % TEST_TOTAL_SHARDS == TEST_SHARD_INDEX
5. Runs only the tests assigned to the current shard using --exact

Usage

rust_test(
    name = "my_test",
    srcs = ["my_test.rs"],
    shard_count = 4,
    experimental_enable_sharding = True,
)