Migration Permissions (#422)

When creating the DBOS system tables with `dbos migrate` you can now
specify an "application role."
If you specify this role, it is granted all permissions needed to run a
DBOS app.

```
 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]                            │
│ --app-role    -r      TEXT  The role with which you will run your DBOS │
│                             application                                │
│                             [default: None]                            │
│ --help                      Show this message and exit.                │
╰────────────────────────────────────────────────────────────────────────╯
```

This is most useful in settings where the application runs with an
"application role" that does not have sufficient privileges 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, specifying the application role to which to grant permissions on
those tables. Then, the application can safely run with the application
role.