feat(rust): make strip_level configurable via nixpkgs_rust_configure

[pcj]

In certain configurations it is necessary to disable stripping. This PR makes this a new optional attribute that is threaded through to the generated rust_toolchain. In the default case, the string "None" is rendered, which is the current implicit default.

Example:

nixpkgs_rust_configure(
    name = "nix_rust",
    default_edition = "2018",
    repository = "@nixpkgs",
    strip_level = {
        "dbg": "none",
        "fastbuild": "none",
        "opt": "none",
    },
)