Add buildpack homepage information to the "io.buildpacks.build.metadata" label on app images produced by lifecycle

[djoyahoy]

Currently, the metadata located under the io.buildpacks.build.metadata label contains buildpack id and version on app images produced by the lifecycle. We reference this metadata to show users what buildpacks were used to produce their images. However, users don't always know where to go to get more information on a specific buildpack. Therefore, we'd like to add homepage to that metadata so we can show users where to look to get more information.

[jromero]

Relevant feedback provided through other means:

• "[User's] initial reaction is to build the image and then inspect it afterwards rather than inspect the resources (stack, store, builder) that worked together to produce the build"
• As a platform provider, I would like to provide better insight into how any particular image was constructed and this should include a reference to where an individual may find information about the particular buildpacks used.