Cli graceful db url (#383)

Get the database URL to use for the DBOS application.

    Order of precedence:
    - If the `db_url` argument is provided, use it.
- If the `dbos-config.yaml` file is present, use the `database_url` from
it.
    - If the `DBOS_DATABASE_URL` environment variable is set, use it.

Otherwise fallback to the same default Postgres URL than the DBOS
library.
Note that for the latter to be possible, a configuration file must have
been found, with an application name set.