Add CLI flag or environment variable for specifying firestore collection for migration records

[jderry-lucem]

The collection used for storing previous migration runs is hard coded as 'fireway'. This creates issues when you want to have multiple migrations for different services in the same project.

One option is to add a flag like:
--migrationsCollection service-fireway. This is similar to golang-migrate which has a config option of migrationsTable.