Add pre and post upgrade step validation feature to extensions API

[fabiocarvalho777]

There should be a pre and a post upgrade step validation, as part of the extensions API.

The pre validation will happen right before each upgrade step, and the post validation will happen right after each upgrade step.

Requirements:

1. Pre-validation: checks if the application is eligible to go through the upgrade step, and fails the transformation if the application is not in a valid state
2. Post-validation: checks if the application is in a valid and acceptable state after the upgrade step transformation. Depending on the validation criteria (which is defined by the extension) the upgrade can proceed to the next upgrade step or fail the whole upgrade, even after all transformation operations have been already performed.
3. Upgrade step validations API is part of Extensions API
4. Upgrade step pre and post validations can be turned ON or OFF
5. Upgrade step pre and post validations should be able to run TUs
6. Upgrade step pre and post validations must not be able to run TOs, or modify the app in any ways (except post-transformation manual instructions)
7. Upgrade step post validations can be used to gather meta-data about the transformed application and produce one ore more post-transformation manual instruction, depending on the state of the transformed application.