Add support for pre-publish hooks for publishing first party Flutter packages

[kenzieschmoll]

Background
A DevTools extension for package:shared_preferences is being added in flutter/packages#6749. The pre-built Flutter web assets of this DevTools extension must be included in the shared_preferences package prior to publishing. However, we do not want an uncontrolled machine to build the extension and check in the output, which means we need a way for this to be done on a controlled machine as a pre-publish step.

Feature request
Add support for a pre-publish hook so that we can support use cases like the one described above. Upon publishing the shared_preferences package, the pre-publish hook would run and build the shared_preferences DevTools extension when needed.

@stuartmorgan please feel free to add any context I may have missed or add suggestions on how you believe this should be implemented.