Two fixes to the batch api route

1. Ensures threadpool worker environment variables parsed as integers (previously caused error since was parsed as strings)
2. Created BatchBaseQueryParams class, rather than relying on BaseQueryParams. Otherwise, if DATABRICKS_SERVING_ENDPOINT is not defined, the batch route required an overall business_unit, data_security_level etc which did not make sense (as these were individually defined in each request within the batch).