flux diff artifact: Provide a non-semantic diff option.

Artifacts may contain other files types, not just YAML files, meaning the
semantic YAML diff provided by `dyff` is not a safe default.

This change implements purely textual diffing using the `diff` command line
tool. This tool can be overridden by users using the `FLUX_EXTERNAL_DIFF`
environment variable.

Users that store Kubernetes resource manifests in the artifact can re-enable
the semantic YAML diff behavior using the `--semantic-diff yaml` flag.

The arguments to the diff subcommand may be:

* A directory
* A .tar.gz or .tgz file
* An OCI url
* An individual file

The two arguments to the command are treated the same way, allowing users to
diff in either direction.

Signed-off-by: Florian Forster <[email protected]>

Author: octo