[Custom Processors] Add variant starting_version field to config for when a version is already in the DB

[banool]

Description

Currently we have only one starting_version field. This field applies unconditionally, even if there is a version in the DB. This means if the user wants to start from the version in the DB if present, but otherwise start from a given version, there is no way for them to do that without a tricky order of starts and stops with adding and removing starting_version from the config.

We should have two fields (more concise naming TBD):

• starting_version_if_nothing_in_db: If given, start from this version if nothing is in the DB.
• starting_version_no_matter_what: Start from this version even if there is something in DB.