feat(oci_pull): expose the digest as a file

[aignas]

This makes the external image repos also expose the digest file, similarly how
the oci_image creates a .digest target. The actual name of the file can be
something else and I am just putting this out to initiate a discussion.

Let's say a user has @alpine image defined via oci_pull, they can access the
digest of the upstream image by accessing the @alpine//:image.digest file.
We could similarly have the platforms of the image written to a file as well,
but I am not sure if that is useful.

Having the image.digest in a file could be useful for various automation that
is done using write_source_file rule where one would use a genrule to get
the digest value from the @alpine//:image.digest target, transform it and
output it to a file that is later synced with write_source_file. Not needing
to depend on extra CLIs that might not be available to the user of rules_oci
as toolchains could be good here.

[alexeagle]

Nice, do you mind also adding some usage of it in the e2e or examples, so we have some form of test coverage that we don't drop or break this in the future?

[thesayyn]

@aignas do you want to finish this PR?

[aignas]

Could you please give some guidance on where the tests should go and if I am expected to bump the lock files?

I would still love the feature, but if someone would like to take over and finish this PR, I am OK with that.

[aignas]

The tests are now passing, @thesayyn, let me know what is left to do here:

$ bazel test :test_bzlmod
INFO: Analyzed target //:test_bzlmod (125 packages loaded, 11031 targets configured).
INFO: Found 1 test target...
Target //:test_bzlmod up-to-date:
  bazel-bin/test_bzlmod_bats.sh
INFO: Elapsed time: 210.224s, Critical Path: 151.69s
INFO: 32 processes: 10 internal, 16 darwin-sandbox, 6 local.
INFO: Build completed successfully, 32 total actions
//:test_bzlmod                                                           PASSED in 64.0s

Executed 1 out of 1 test: 1 test passes.
There were tests whose specified size is too big. Use the --test_verbose_timeout_warnings command line option to see which ones these are.

$ bazel test :test_nobzlmod
INFO: Analyzed target //:test_nobzlmod (0 packages loaded, 1 target configured).
INFO: Found 1 test target...
Target //:test_nobzlmod up-to-date:
  bazel-bin/test_nobzlmod_bats.sh
INFO: Elapsed time: 69.553s, Critical Path: 66.46s
INFO: 6 processes: 4 internal, 2 local.
INFO: Build completed successfully, 6 total actions
//:test_nobzlmod                                                         PASSED in 66.2s

Executed 1 out of 1 test: 1 test passes.
There were tests whose specified size is too big. Use the --test_verbose_timeout_warnings command line option to see which ones these are.