Add pre and post transformation validation feature to extensions API

[fabiocarvalho777]

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

In case of upgrades, the pre validation will happen only once, right before the first upgrade step, and the post validation will happen only once as well, right after the last upgrade step.

Requirements:

1. Pre-validation: checks if the application is eligible to be transformed, and fails the transformation before it even starts if the application is not valid
2. Post-validation: checks if the application is in a valid and acceptable state after the transformation. Depending on the validation criteria (which is defined by the extension) the whole transformation can be declared as success or fail, even after all transformation operations have been already performed.
3. Transformation validations API is part of Extensions API
4. Pre and post transformation validations can be turned ON or OFF
5. Pre and post transformation validations should be able to run TUs
6. Pre and post transformation validations must not be able to run TOs, or modify the app in any ways (except post-transformation manual instructions)
7. Post transformation 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.