Adding select statements for build script environment and data attributes in bzlmod (#3511)

An implementation to bring some select statement functionality which are
available in [repository
rules](https://github.com/bazelbuild/rules_rust/blob/f04b2a6f9dc3f26e29563867155da8577b7203ad/crate_universe/private/crate.bzl#L258)
but not bzlmod.

Since a dict of dicts is not supported, I've opted for a dict of json
string which gets decoded on the other end.