Add annotation_selects tag class

[finn-ball]

Adds an annotation_select tag class.

This annotation will add common environment variables:

crate.annotation(
    crate = "foo",
    build_scipt_data = [
        "@bar",
    ],
    build_script_env = {
        "BAR": "bar",
    },
)

This annotation will append a platform dependent select statement to the above variables:

crate.annotation_select(
    crate = "foo",
    triples = ["x86_64-unknown-linux-gnu"],
    build_script_data = [
        "@baz",
    ],
    build_script_env = {
        "BAZ": "baz",
    },
)

Attempts to improve #3511

[illicitonion]

Sorry for the delay here!

LGTM - can you patch in (or otherwise achieve) illicitonion@1e5a5ac to error rather than override on duplicate triple setting?