Provide project source metadata from project.toml

[jromero]

Description

As a buildpacks user, I'd like to have the source and version information from my project.toml file embedded as metadata in the final app image.

Proposed solution

Based on the spec, we should be able to provide <layers>/project-metadata.toml in this format which would be translated into the io.buildpacks.project.metadata label on the image.

This issue should be scoped to project.toml information.

Acceptance Criteria

General case

Given I have a `project.toml` with the following contents:
  [project]
  version = "1.0.2"
  source-url = "https://github.com/buildpacks/pack"
And I run `pack build my-app`
When I run the following command `docker inspect my-app | jq -r '.[0].Config.Labels["io.buildpacks.project.metadata"] | fromjson'`
Then I should see:
{
  "type": "project",
  "version": { "declared": "1.0.2" },
  "metadata": {
    "url": "https://github.com/buildpacks/pack"
  }
}

Additional context

This issue is part of an epic of work. The related issues may be see by filtering on epic/source-metadata.