refactor: image tag flagging process

[davidhsingyuchen]

Part of #1680

• Create a file in pkg/api/types/${cmd}_types.go, and define the CommandOption for this command
• Create some file in pkg/cmd/${cmd}, and move the command entry point in real into this package

Notes:

• Also renamed cmd/nerdctl/tag.go to cmd/nerdctl/image_tag.go because it's the canonical name. Please let me know if it should be done in another PR instead.
• The field names of ImageTagCommandOptions (i.e., Source and Target) come from the description of the command.
• Noticed that cmd/nerdctl/images.go is parsing the image references, but that should be because the filters need to be created based on those references as it makes sense to let image.List() only accept filters as an uniformed interface. As a result, I left the image reference parsing code in pkg/cmd/image/tag.go instead.

Signed-off-by: Hsing-Yu (David) Chen davidhsingyuchen@gmail.com

[AkihiroSuda]

Thanks