Support Platform API 0.13

[natalieparellano]

Description

See spec milestone: https://github.com/buildpacks/spec/milestone/27

• Platform API changes for image extension build contexts
  • This function can no longer rely on the presence of a "build" directory to indicate build extensions - instead, we must traverse the directory and look for a <dir>/<buildpack-id>/build.Dockerfile
• Making builpack image extensions feature non-experimental
  • Let's remove the guard in pack build
  • We don't have to set the env var here, here, here, and here anymore
• [RFC #0111] Support insecure registries - see other issue
• [RFC 0125] Lifecycle parallel export - maybe it doesn't make sense to expose this in pack just set; we might wait until this feature is requested

Proposed solution

See above

[sarthaksarthak9]

I would love to work on this issue @natalieparellano

[sarthaksarthak9]

@natalieparellano do we have to implement all changes in a single pr ?

[natalieparellano]

@sarthaksarthak9 it could be done in separate PRs. Happy to provide further guidance if you're still interested in working on this.

[sarthaksarthak9]

I would love to work @natalieparellano