Add dependency constraint metadata to buildpack.toml

[sophiewigmore]

In issue paketo-buildpacks/packit#140, @ryanmoran outlined a full plan to manage dependency updates in a more transparent manner using the dependency server. We are breaking that issue down into workable pieces.

For this initiative, we are going to implement this work in the Node.js language family first, before we roll out the changes to every buildpack. This issue is to add the constraint metadata fields to the buildpack.toml for the node-engine buildpack.

As described in paketo-buildpacks/packit#140 the format should look like:

[[metadata.dependency-constraints]]
  id = "noder"
  constraint = "12.*"
  patches = 2

[[metadata.dependency-constraints]]
  id = "node"
  constraint = "14.*"
  patches = 2

[[metadata.dependency-constraints]]
  id = "node"
  constraint = "15.*"
  patches = 2

[[metadata.dependency-constraints]]
  id = "node"
  constraint = "16.*"
  patches = 2

This will eventually get used by our automation to keep the versions in the buildpack.toml up to date. Please refer to paketo-buildpacks/packit#140 for more information on this schema.