Add a new `Resolve` method to `Client` that will resolve its `Target`
to the corresponding manifest digest id and its corresponding iamge
identifier. The former can be used in the URL to fetch a specific
image from the registry via `<name>@<digest>` and the latter uniquely
identifies a container image via the hash of its configuration object.
This should stay the same across pulls and is also the id returned via
`podman pull` and `podman images`.
Since (most) container images are OS and architecture specific a tag
often points to a manifest list that contains all available options.
Therefore the resolve operation needs to choose the correct arch for
image. A new pair of getters `Set{Architecture,Variant}Choice` lets
the user control which architecture/variant is selected during the
resolution process.
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