[RFC #0101] Add System Buildpack to Platform API

[ekcasey]

RFC 0101 - System Buildpacks in Builder Images

Adds system.toml to platform API.

/cnb/lifecycle/detector \
  [-system <system>]\

Where:

• the lifecycle SHALL merge the pre group with each group from <system> such that the pre buildpacks are placed at the beginning of each order group before running detection.
• SHALL merge the post group with each group from <system> such that the post buildpacks are placed at the end of each order group before running detection.

system.toml (TOML)

[[system.pre.buildpacks]]
  id = "<buildpack ID>"
  version = "<buildpack version>"
  optional = true

[[system.post.buildpacks]]
  id = "<buildpack ID>"
  version = "<buildpack version>"
  optional = true

[mboldt]

Could we target this for Platform 0.10 since it ties into #245 through the .profile buildpack?

Happy to put the above in a PR.

[natalieparellano]

We've wanted to implement this for awhile now... I'm putting it in Platform 0.14 so as not to block other (smaller) efforts in Platform 0.13.