[VR-12269] Enable alerter to propagate store_aggregate for finding samples

[convoliution]

Problem

If an alert is defined with summary_sample_base fields that aren't explicitly supported by the client, those fields end up getting dropped when alert.summary_sample_base is accessed because of SummarySampleQuery's implementation.

Solution

Rather than have a bunch of explicit conversion logic in SummarySampleQuery._from_proto_request() and SummarySampleQuery._to_proto_request(), I've changed SummarySampleQuery to transparently carry an protobuf message. I've listed some alternative solutions I considered in a Google Doc linked on the ticket.

Tests

$ pytest monitoring/
==================================================== test session starts ====================================================
platform darwin -- Python 3.7.10, pytest-6.2.4, py-1.10.0, pluggy-0.13.1
rootdir: /Users/miliu/Documents/modeldb/client/verta/tests, configfile: pytest.ini
plugins: hypothesis-6.14.0
collected 70 items                                                                                                          

monitoring/test_aggregation.py ..                                                                                     [  2%]
monitoring/test_client.py ..                                                                                          [  5%]
monitoring/test_profilers.py ....s                                                                                    [ 12%]
monitoring/test_queries.py .                                                                                          [ 14%]
monitoring/test_summaries.py ...                                                                                      [ 18%]
monitoring/test_summary_samples.py ..                                                                                 [ 21%]
monitoring/alerts/test_alerter.py ........                                                                            [ 32%]
monitoring/alerts/test_entities.py .............                                                                      [ 51%]
monitoring/alerts/test_status.py .........                                                                            [ 64%]
monitoring/comparison/test_comparison.py ..................                                                           [ 90%]
monitoring/notification_channels/test_config_classes.py ..                                                            [ 92%]
monitoring/notification_channels/test_entities.py .....                                                               [100%]

================================== 69 passed, 1 skipped, 43 warnings in 166.94s (0:02:46) ===================================