Configurable Env Vars and Post Installation Commands from project.toml

[tiff-ritz]

These changes collectively enhance the buildpack's flexibility and usability, allowing for more sophisticated configuration and automation during the build process.This pull request introduces several significant changes to the buildpack configuration and package installation process. The main changes include adding support for configuring environment variables and commands for installed packages, updating dependencies, and enhancing the test coverage.

New keys were added to the project.toml file to allow the ability to configure package environment variables and post installation commands.

env - a dict {} of key/pairs defining environment variables to be exported to the environment after package installation
commands - a list of string commands to be executed after package installation. These will be executed in the order they are listed.

During the build process, the buildpack will read the project.toml file and apply the specified environment variables. The {install_dir} placeholder will be replaced with the actual paths so the variables are available at both build and launch phases using [layer environment variables][cnb-environment].

The buildpack includes a set of default environment variables for each package, known as PACKAGE_ENV_VARS. These default environment variables are applied during the build process. However, you can override these default values by specifying environment variables in the project.toml file.

If a package is listed in the project.toml file with environment variables under the env key, those variables will take precedence over the default PACKAGE_ENV_VARS.

[_]
schema-version = "0.2"

[com.heroku.buildpacks.deb-packages]
install = [
    # string version of a dependency to install
    system-cron,
    # inline-table version of a dependency to install
    { name = "git", env = { "GIT_EXEC_PATH" = "{install_dir}/usr/lib/git-core", "GIT_TEMPLATE_DIR" = "{install_dir}/usr/share/git-core/templates" }, commands = ["echo 'Git installed successfully'", "git --version"]},
    { name = "babeld" },
    { name = "ghostscript", skip_dependencies = true, force = true, env = { "GS_LIB" = "{install_dir}/var/lib/ghostscript" }, commands = ["echo Ghostscript installed successfully", "gs --version"]},
]

Added unit tests to:

buildpack_config.rs
requested_package.rs
environment.rs
install_packages.rs

Integration Tests added:

test_get_integration_test_builder()
test_get_integration_test_arch()
test_panic_unsupported_test_configuration()
test_set_install_config()