Add custom label metadata to packaged buildpacks #1877

Introduces a new flag `--label` to the `pack buildpack package` command which: * can be used to add labels one at a time (e.g.; `--label a=1 --label b=2`) * can be used to add multiple labels in CSV format (e.g.; `--label a=1,b=2`) The labels are parsed into a `map[string]string` and added as label metadata to the packaged OCI or `.cnb` file. Signed-off-by: Colin Casey <[email protected]>