Spike for supporting buildplan configuration

[colincasey]

Warning

This is just an experiment. Do not merge!

Use Case

Let's say you're writing a CNB and, for the functionality your buildpack provides, you need to ensure one or more system packages are installed. Great! You can use heroku/deb-packages for that! One tiny problem though, the heroku/deb-packages CNB only supports configuration via project.toml. This means that your buildpack has to instruct users to manually add the required configuration for the packages it requires or it won't work. Wouldn't it be nice if your buildpack could instead just contribute the required configuration via the buildplan?

Solution

This PR exposes the same configuration structure that available to end users in project.toml to buildpack authors in a buildplan. To leverage this configuration, a participating CNB needs to do the following in the detect phase:

• require the heroku/deb-packages buildplan and provide all necessary configuration under the metadata field.

Example

detect.sh

#!/usr/bin/env bash
build_plan="$2"

cat <<EOF >"$build_plan"
[[requires]]
name = "heroku/deb-packages"

[requires.metadata]
# add custom packages
install = ["mongodb-org-tools", "mongodb-org-shell"]

# register a custom source
[[requires.metadata.sources]]
uri = "https://repo.mongodb.org/apt/ubuntu"
suites = ["noble/mongodb-org/8.0"]
components = ["multiverse"]
arch = ["amd64", "arm64"]
signed_by = """-----BEGIN PGP PUBLIC KEY BLOCK-----

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=J31U
-----END PGP PUBLIC KEY BLOCK-----
"""
EOF