Can we support custom extensions?

[sophiewigmore]

Context

Issue #355 brought up the need for allowing the usage of extensions outside of the set that are shipped with the distribution of PHP. Right now, there's no way to do this unless we add a new extension to the list we include in the PHP dependency in the buildpacks. This is less than ideal because it could become a never-ending cycle of adding extensions in a one-off manner whenever someone needs one.

Issue

With paketo-community/explorations#8 on the horizon, it might be good to consider if we can find a way to allow for a more flexible set of extensions for PHP. Whether this is during the PHP compilation process, or delegated to a buildpack we should investigate how to make this possible.

Outcome

We have a separate issue that outlines a tangible way a user could use non-default extensions with their PHP app when using the PHP CNB.