Provide an option to change "CNB_PLATFORM_EXPERIMENTAL_FEATURES" setting

[jromero]

Description

As per buildpacks/rfcs#115, we'll likely want to provide a mechanism to change the ability to tweak the CNB_PLATFORM_EXPERIMENTAL_FEATURES setting.

Proposed solution

A few options:

• Add a flag to build (ie. --experimental={warn|silent|error})
• Add an option to config.toml to globally set value
• Roll it up into pack's existing experimental=true in config.toml to equal "silent" otherwise "error"

Additional context

We should make sure it's clear, if we separate them, that there is a distinction between pack and lifecycle experimental features.

[jjbustamante]

Closing this one, we ended up implementing option 3 with this PR