portable: add flag to return extension-releases in GetImageMetadataWithExtensions

portable: add flag to return extension-releases in GetImageMetadataWithExtensions

Return the name of each extension and the associated extension-release
file, and pretty-print them in 'portablectl inspect', if a new flag
is passed.

$ portablectl inspect --extension app2 --extension app0  minimal app0 app1
(Matching unit files with prefixes 'app0', 'app1'.)
Image:
        /run/portables/minimal.raw
Portable Service:
        n/a
Operating System:
        Debian GNU/Linux 10 (buster)
Extension:
        /run/portables/app2.raw
        Extension Scope:
                n/a
        Extension Compatibility Level:
                n/a
        Portable Service:
                n/a
        Portable Prefixes:
                n/a
        Operating System:
                n/a (debian 10)
Extension:
        /run/portables/app0.raw
        Extension Scope:
                n/a
        Extension Compatibility Level:
                n/a
        Portable Service:
                n/a
        Portable Prefixes:
                n/a
        Operating System:
                n/a (debian 10)
Unit files:
        app0.service