[Idea]: Bootstrap bazelisk

Today the `ctx.bazel` machinery expects that the user has a `bazel` on their `$PATH` somewhere. Ideally the Aspect CLI would do the right thing and mostly stand alone, which means provisioning a `bazelisk` transparently by default.

- Add a new `cache.axl` implementation transliterating the current `cache` crate
- Add a new `bazelisk.axl` implementation which consumes the current HTTP feature(s) and the `cache` package to provide a helper for downloading a `bazelisk` binary
- Extend the reference `config.axl` to use the `bazelisk.axl` and install a `bazel` binary into the config struct.
- Extend the `ctx.bazel` machinery to go through the config struct to get the path of the `bazel` nominal binary.