Expose EventCount via api

[greenape]

• Add marshmallow spec for EventCount
• Add to allowable joined aggregate queries
• Add flowclient function event_count(*, start, stop, direction, event_types, subscriber_subset)
• Add test to integration_tests/tests/test_queries.py
• Update acceptance tests
  • integration_tests/tests/flowapi_tests/test_api_spec.py
  • integration_tests/tests/flowmachine_server_tests/test_server.py
• Update Changelog

event_types should in schema be EventTypes(), and take None being passed into the client function as meaning 'all' when it actually gets executed as with LocationEvenCounts