Import mechanism for existing MLFlow databases

[jgiannuzzi]

We should provide an import command that allows to import an existing MLFlow database (sqlite or postgres) into an FML database.

When opening an existing an MLFlow database, we currently migrate it, but some users may choose to keep it as-is and instead import it into a new FML database. Another use case is going from one DB backend to another, e.g. migrating from sqlite to postgres.

The import command should take a source and a destination URI. In the future, we may want to support other source formats (e.g. MLFlow directory, Tensorboard, etc.) but this issue is only about MLFlow sqlite and postgres databases.

• [x] #181
• [ ] #182