Command to create a tag for a manifest in the remote registry

[qweeah]

The proposed command is:

Usage:
  oras tag  REPO:<TAG|DIGEST> NEW_TAG

Flags:
  --ca-file string               server certificate authority file for the remote registry
  -d, --debug                    debug mode
  -h, --help                     help for tagging
      --insecure                 allow connections to SSL registry without certs
  -p, --password string          registry password or identity token
      --password-stdin           read password or identity token from stdin
      --plain-http               allow insecure connections to registry without SSL check
  -u, --username string          registry username
  -v, --verbose                  verbose output

Relate to #459