Extend Applier's Apply() method with an optional options parameter

Extend the Applier interface's Apply method with an optional
options parameter.

For the container image encryption we intend to use the options
parameter to pass image decryption parameters ('dcparameters'),
which are primarily (privatte) keys, in form of a JSON document
under the map key '_dcparameters', and pass them to the Applier's
Apply() method. This helps us to access decryption keys and start
the pipeline with the layer decryption before the layer data are
unzipped and untarred.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Harshal Patil <harshal.patil@in.ibm.com>

diff.go

@@ -45,9 +45,15 @@ type diffRemote struct {
 	client diffapi.DiffClient
 }
 
-func (r *diffRemote) Apply(ctx context.Context, diff ocispec.Descriptor, mounts []mount.Mount) (ocispec.Descriptor, error) {
+func (r *diffRemote) Apply(ctx context.Context, desc ocispec.Descriptor, mounts []mount.Mount, opts ...diff.ApplyOpt) (ocispec.Descriptor, error) {
+	var config diff.ApplyConfig
+	for _, opt := range opts {
+		if err := opt(&config); err != nil {
+			return ocispec.Descriptor{}, err
+		}
+	}
 	req := &diffapi.ApplyRequest{
-		Diff:   fromDescriptor(diff),
+		Diff:   fromDescriptor(desc),
 		Mounts: fromMounts(mounts),
 	}
 	resp, err := r.client.Apply(ctx, req)