Updates platform spec for stack metadata RFC

[kvedurmu]

RFC - buildpacks/rfcs#78

[nebhale]

There's still an outstanding requirement for a label that contains any extensions/metadata that a stack author would like to add, that isn't in this list.

[nebhale]

@kvedurmu I'm going to move this to Draft until the RFC passes.

[kvedurmu]

Makes sense @nebhale. Addressed the final comment around adding the metadata label. The one thing that wasn't clear for this field is whether this should be defined in the spec as SHOULD or MAY, but I'll leave that up to the core team to decide. Other than that the PR should be good to merge once the RFC passes.

[ekcasey]

Worth noting that once #87 lands this PR will need to be updated with new rebase requirements. Rebase should ensure that io.buildpacks.stack.* labels on app image match the labels on the new run image.

[nebhale]

@kvedurmu This is now clear for @buildpacks/core-team review. You'll want to take forthcoming feedback and incorporate it.

[ekcasey]

Repointed at platform/0.5

[kvedurmu]

@ekcasey @nebhale updated to include the rebasing requirements. Let me know if there's anything else I missed.

[nebhale]

@kvedurmu as part of today's discussion the following two updates have been suggested:

1. Change the name of io.buildpacks.stack.release_date to io.buildpacks.stack.released
2. Remove io.buildpacks.stack.version. Stacks are comprised of two images, with different version numbers. This may be re-added with an image-type specific tags, but stacks themselves do not have a single version number.