Add get_layer_info_json

[RishabhSaini]

This API exposes the "GetLayerInfosForCopy" API in containers/image,
which can be used to create a mapping between blob and diff IDs,
allowing pulling via diffID.

Signed-off-by: RishabhSaini rsaini@redhat.com

[cgwalters]

Let's provide a bit more documentation, something like:

/// Returns data that can be used to find the "diffid" corresponding to a particular layer.

to start.

[cgwalters]

Also two other things:

• Can you add some content into your commit messages? Even something minimal like:

Add get_layer_info_json

This API exposes the "GetLayerInfosForCopy" API in containers/image, which can be used
to create a mapping between blob and diff IDs, allowing pulling via diffID.

• You'll need to sign off on your commit in both this repo and skopeo (the containers/ github organization has a global check)

[cgwalters]

And thanks so much for working on this!

[cgwalters]

OK, this is looking nice and simple as is. Just one nit on the docs. That said...I'm thinking we may need to add a bit more complexity here to handle ostreedev/ostree-rs-ext#396 (comment)

We can have a realtime chat about this but basically...if we choose to go with the "dynamic detection" route, we'd need to dynamically check the protocol version the remote gives us.

See e.g. the code around let protover = semver::Version::parse(protover.as_str())?;

[cgwalters]

Do we still need this?

[cgwalters]

Thanks!