feat: don't produce big tar file as default output of oci_tarball

[alexeagle]

This is expected to be a big performance improvement because it doesn't overload the remote cache with these files, that only should be needed for local dev workflow of load'ing the image into a container runtime.

Outputs

The default output is an mtree specification file.
This is because producing the tarball in bazel build is expensive, and should typically not be an input to any other build actions,
so producing it only creates unnecessary load on the action cache.

If needed, the tarball output group allows you to depend on the tar output from another rule.

On the command line, bazel build //path/to:my_tarball --output_groups=tarball

or in a BUILD file:

oci_tarball(
    name = "my_tarball",
    ...
)
filegroup(
    name = "my_tarball.tar",
    srcs = [":my_tarball"],
    output_group = "tarball",
)

[alexeagle]

@thesayyn we have to figure out whether we want to use tags=manual and if so, whether there's any point splitting the mtree action from the tar action

[illicitonion]

Seems pretty reasonable to me!

[thesayyn]

A small nit, looks great!