Add initial flow aggregation code and API

[caseydavenport]

Description

This PR introduces a new component responsible for ingesting, aggregating, and the servering flow log data from the individual felix daemons deployed in a Calico cluster. This is still an early sketch of the component, but the major functional pieces are in place so it's a good place to start.

It is made of a few parts:

• a gRPC API with which each Felix is able to push flow log data to the aggregator.
• an aggregator component, which groups / aggregates data from across the cluster into time-windowed buckets
• a gRPC API which allows querying of the aggregated data
• an HTTP emitter component, that emits further time-aggregated data to a configurable endpoint.

Related issues/PRs

Todos

• Tests
• Documentation
• Release note

Release Note

TBD

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