Fix check validators signatures when applying forced changes

[dimartiro]

We did re introduce an old issue #3468 since the previous fix #4144 did brake the sync process when we have to apply forced justifications. So to prioritize the sync process we decided to re introduce the issue and address it here.

We have to be able to apply auth changes (scheduled or forced) before executing the block and verifying the justification.

I've been checking substrate code and they are:

1. Applying the authorities changes
2. Importing the block
3. Processing the justification

We want to reproduce that behavior on our side