Add support for cargo vendor

[abrisco]

cargo vendor is something some companies use to either ultra lock down their dependencies or satisfy requirements in their massive monorepos.

My current thought is that vendoring support would be done as a rule or macro which defines a runnable target that would both perform the vendoring and write the BUILD files to the appropriate directories.

load("@cargo_bazel//:defs.bzl", "cargo_bazel_vendor", "crate")

cargo_bazel_vendor(
    name = "cargo_bazel_vendor",
    manifest = ":Cargo.toml",
)

Users could then run bazel run //:cargo_bazel_vendor to vendor crates into their workspace and have BUILD files rendered into place. Maybe this rule could even support a packages attribute which allows you to inline the desired crates.

Again, just thoughts, not sure if or when this should be done.