Option to create a config file from `fly pg create`

[fliepeltje]

From a DX perspective it would make sense to me if there was a "declarative" record of the database. I think it'd be a nice feature if you could either by default or by means of a flag have a fly.pg.toml generated.

That way:

• you could manage the app resources through the -c flag rather than the --app flag
• the database is 'visible' as a configuration
• the mental model for what a database actually is underneath is a bit easier