Refactor signature module from function way to object oriented. Also
changed the logic a little to avoid duplicated get resource operation.

Signed-off-by: Xynnn007 <[email protected]>

change the whole resource module's API into an object. This would help
to make the modularation clear and better maintainance.

Also, change the feature `getresource` to `kbs`. Because this feature
only adds supports to find resources with `kbs:///` uri scheme.

deletes useless lines in Cargo.toml

Signed-off-by: Xynnn007 <[email protected]>

Now auth is a separate module with an object. This would help to
maintain the internal status of each auth module.

Signed-off-by: Xynnn007 <[email protected]>

With clientBuilder, we can get registry auth, sigstore config file and
image policy when the client is built.

This would help to avoid duplicated file getting.

Also, combined CDH's image config with image-rs'.
To leverage the new client builder, we mark the image pull object as a
static one to be lazily initialized. Because before CDH is set up, the
initialization steps of image client cannot pass when fetching resources
from KBS.

Signed-off-by: Xynnn007 <[email protected]>

Signed-off-by: Xynnn007 <[email protected]>

Now we use kbs to refer to the old feature getresource

Signed-off-by: Xynnn007 <[email protected]>

New dependency oci-client version supports to set HTTPS_PROXY and
NO_PROXY without setting env of the whole process.

Also, this commit marks the cosign module's client TODO to add a proxy
configuration.

Signed-off-by: Xynnn007 <[email protected]>

Fixes confidential-containers#525

Signed-off-by: Xynnn007 <[email protected]>