System Database URL (#442)

This PR enables configuring DBOS with just a system database, like this:

```python
config: DBOSConfig = {
    "name": "dbos-starter",
    "system_database_url": os.environ.get("DBOS_SYSTEM_DATABASE_URL"),
}
DBOS(config=config)
```

If using transactions, the application database can be configured
separately:

```python
config: DBOSConfig = {
    "name": "dbos-starter",
    "system_database_url": os.environ.get("DBOS_SYSTEM_DATABASE_URL"),
    "application_database_url": os.environ.get("APP_DATABASE_URL"),
}
DBOS(config=config)
```

This is fully backwards-compatible—the old way of configuring DBOS
with just `database_url` (which points to an application database, with
the system database name suffixed with `_dbos_sys` if not provided) is
still supported, though it will be removed in a future major release.

The client interface and CLI are updated similarly.

The client can now be created with just a system database URL:

```python
    client = DBOSClient(
        system_database_url=os.environ.get("DBOS_DATABASE_URL"),
    )
```

All CLI commands now accept a `--sys-db-url` parameter.