Better "dbos migrate" CLI (#419)

This command enables using the `dbos migrate` CLI command to create DBOS
system tables. Usage is:

```
 Usage: dbos migrate [OPTIONS]

 Create DBOS system tables.

╭─ Options ─────────────────────────────────────────────────────────────────────────────╮
│ --db-url      -D      TEXT  Your DBOS application database URL [default: None]        │
│ --sys-db-url  -s      TEXT  Your DBOS system database URL [default: None]             │
│ --help                      Show this message and exit.                               │
╰───────────────────────────────────────────────────────────────────────────────────────╯
````

This is most useful in settings where the application does not run with
sufficient privilege to create the DBOS system database or system
tables. Prior to deployment, the `dbos migrate` command can be run with
a privileged user to create DBOS system tables. Then, the application
can run with a less privileged user that has access to the system
database but does not need CREATE privileges.