[01-postgres] Add instructions to allow-list the postgis extension

Error via Microsoft Open Source Discord:

```
File ".\azure-python-labs\01-postgres\pg-lab.py", line 30, in loadData
cursor.execute('CREATE EXTENSION IF NOT EXISTS postgis')psycopg2.errors.FeatureNotSupported: extension "postgis" is not allow-listed for "azure_pg_admin" users in Azure Database for PostgreSQL
HINT: to see the full allow list of extensions, please run: "show azure.extensions;"
```

The extension will need to be allow-listed per https://docs.microsoft.com/en-us/azure/postgresql/flexible-server/concepts-extensions#how-to-use-postgresql-extensions