Openeo-aggregator

- Add config option `job_options_update` to inject job options before sending a job to upstream back-end ([135](https://github.com/Open-EO/openeo-aggregator/issues/135))

- Remove (now unused) `AggregatorConfig` class definition ([112](https://github.com/Open-EO/openeo-aggregator/issues/112))

- Remove deprecated `AggregatorConfig` fields: `aggregator_backends`, `partitioned_job_tracking`,
`zookeeper_prefix`, `memoizer` ([112](https://github.com/Open-EO/openeo-aggregator/issues/112))
- Remove `AggregatorConfig` usage (both in `src` and `tests`). Remove support for `OPENEO_AGGREGATOR_CONFIG` based configuration ([112](https://github.com/Open-EO/openeo-aggregator/issues/112))

- Remove now unused `conf/backend_config.py` ([112](https://github.com/Open-EO/openeo-aggregator/issues/112), [#117](https://github.com/Open-EO/openeo-aggregator/issues/117))

- Add `AggregatorBackendConfig.memoizer` and deprecate `AggregatorConfig.memoizer` ([112](https://github.com/Open-EO/openeo-aggregator/issues/112))

- Add `AggregatorBackendConfig.partitioned_job_tracking` and deprecate `AggregatorConfig.partitioned_job_tracking` ([112](https://github.com/Open-EO/openeo-aggregator/issues/112))