make sdbmigrate convenient to use with cloudy PostgreSQL

[drednout]

User should be able to run sdbmigrate with dynamic configs, received from HTTP API or other external source. Also it would be great to provide ability to run migrations with Python import.

Some examples of cloud PostgreSQL:

1. neon.tech provides connection string:
   postgres://LOGIN:PASSWORD@ep-jolly-disk-96330411.eu-central-1.aws.neon.tech/neondb
2. AWS RDS provides all creds separated, buty they should be stored in secret manager or Consul:
   https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/CHAP_GettingStarted.CreatingConnecting.PostgreSQL.html