Generate Migrations command: Support updating existing tables

[edalzell]

My use case:

• existing resource/table
• added 30 fields to blueprint
• need to create new migration

Ended up pointing site at new db, then generating migration for the resource, then changing that migration to update the table.

[duncanmcclean]

Expanding on this... we could probably have Runway listen to the BlueprintSaved event and generate a migration based on the diff of changes in the blueprint.

We might not even need the command anymore if it's all driven by listeners.

[duncanmcclean]

I'm going to close this in favour of #519, which will remove the need for the existing generate-migrations command.