Migration for external Audit DB

[plettich]

In case the audit log is send to a different database using PI_AUDIT_SQL_URI, possible migrations of the audit table will not be applied.
We should check for the existence of that configuration in the pi.cfg and either apply the migrations to the external table as well (need to check how to use a different db in alembic) or give a note to the user to apply the changes themselves.

[cornelinux]

@plettich Which CC is it?

[plettich]

@cornelinux

@plettich Which CC is it?

You can search for the issue number in the ticketing system. I try to avoid posting internal data here.

[cornelinux]

We add docs:

• in the description within pi.cfg
• in the READ-BEFORE-UPDATE
  • a generic comment
  • comments in the upgrade steps, where the audit log schema was changed