Add an option to build OCI bundles with zstd compressed layers.
gzip is kept as the default for maximum compatibility:
Ecosystem support:
distribution/distribution: no explicit support, but works
quay.io: sinc 2021
Amazon ECR: supported
pulp_container: since 2022
flatpak: since first-OCI supporting version
tardiff: since first version
To avoid passing around combinations of a FlaptakOciRegistry with
repository and digest, add a FlatpakImageSource type.
This also reduces duplicated code where every place that did
this independently retrieved the repository and image config.
