Add support for alpine's /lib/apk/db/installed

[kpcyrd]

$ osv-scanner --lockfile /lib/apk/db/installed 
could not determine parser for /lib/apk/db/installed

It seems osv.dev has alpine data available so it seems like a good match. :)

[questgugou]

thankyou verymuch

[cmaritan]

Hello @oliverchang and @another-rex ,

In this commit I've added support for apk "installed" file. I'm adding some error management, tests and will soon make a PR

Regards.