Record/display the Buildpack API version for each buildpack

[edmorley]

As time goes by, we're going to end up in a situation where a proportion of the buildpacks listed on the registry are using an old and no longer supported Buildpack API version, and can't be used by modern versions of lifecycle, Pack CLI/other platforms etc.

When we reach that point, it will be incredibly helpful if we know the buildpack API version of all of the buildpack-version combinations, so we can do things like hiding unsupported buildpacks.

As a first step, we should start recording the Buildpack API version in the DB and displaying it on the listings page for each buildpack.