Write a Bazel module extension to download the `cosmos` binaries.

We currently list all the binaries, integrities and repository rules in MODULE.bazel.

This has been sufficient for the most part but now that we are doing more regular updates it is becoming a challenge to maintain.

I propose writing a extension that can be used as so:

cosmos = use_extension("//ape/cosmos:defs.bzl", "ape_cosmos")
cosmos.download(
    version = "4.0.2",
    lock = "//ape/cosmos/lock.json",
)
use_repo(cosmo, "zstd")

Where the lock.json contains the binary name and integrity:

{
    "zstd": "sha256-...",
    "chmod.ape": "sha256-...",
    ...
}

It would generate the various download, entrypoint and exported symlink repositories for each binary.

We should implement a way to update the lockfile. An option would be to set the integrity to null and then the module context does a download and updates the integrity.