Bootstrap `crates_universe/extensions.bzl`

[ericmcbride]

Noticed there is no way to "bootstrap" the new cargo features (extensions) in MODULE.bazel like you can for crates_universe. Currently you have to test the extension by using CARGO_BAZEL_GENERATOR_URL and CARGO_BAZEL_GENERATOR_SHA256 environmentals.

Ideally rules_rust crates_universe extension maintains feature parity with crates_universe, to enable developers to have the same experience while developing.