Update crates_vendor to write any crate spec and annotation used

[UebelAndre]

This would allow external consumers of workspaces that use crates_vendor to be programmatically consume the same dependencies.

Concretely, the snippet:

crates_vendor(
    name = "crates_vendor",
    packages = {
        "rand": crate.spec(
            version = "1",
        ),
    },
    annotations = {
        "rand": [crate.annotation(
            default_features = False,
            features = ["small_rng"],
        )],
    },
    mode = "remote",
    vendor_path = "crates",
    tags = ["manual"],
)

To generate some content in the generated crates.bzl file:

PACKAGES = {
    "rand": crate.spec(
        version = "1",
    ),
}

ANNOTATIONS = {
    "rand": [crate.annotation(
        default_features = False,
        features = ["small_rng"],
    )],
}