Add tool to seamlessly migrate Akonadi between database engines

This introduces akonadi-db-migrator CLI tool that allows for simple
migration between supported database engines. It works by creating
a new database using the selected engine and copying all tables
row by row from the old database. This way, IDs are preserved so
that all existing relations as well as all application configurations
that rely on Collection IDs or Item IDs remain valid.

On my sysem with NVMe SSD it takes about 2 minutes to migrate
database with ~300,000 PimItems.