Add support for buildpack.toml targets for Buildpack API 0.10

[joshwlewis]

Buildpack API 0.10 has been released as of August 3, 2023: https://github.com/buildpacks/spec/releases/tag/buildpack%2Fv0.10.

It introduces the concept of buildpack targets, which replace the concepts stacks and mixins:

• https://github.com/buildpacks/spec/pull/336
• https://github.com/buildpacks/rfcs/blob/main/text/0096-remove-stacks-mixins.md