Finalize Platform/0.13

[natalieparellano]

Release notes:

Features

• It is possible to export the app image and cache image in parallel (https://github.com/buildpacks/spec/pull/380)
• The lifecycle accepts insecure registries configuration (https://github.com/buildpacks/spec/pull/385)
• Image extensions are no longer experimental (https://github.com/buildpacks/spec/pull/386)
• Image extensions can define the build context directory for build and run base image extension (https://github.com/buildpacks/spec/pull/384)