Proposal: Flink support

Auron just support Spark(for batch) now. But in terms of capability, it can be used to support Flink (for streaming).

For many user scenarios in Flink, processing at the second or even minute level is perfectly acceptable. Examples include mini-batch processing for unbounded aggregates or minute-level window aggregations.

Auron will build Flink capabilities in three phases.
Phase One: Stateless Operator Support(Calc Operator)
* Support for various operator expressions: such as + - * /
* Support for Flink built-in functions
* Support for user-defined Flink functions

Phase Two: Stateful Operator Support(Agg Operator)

* Support for Flink built-in agg functions
* Support for user-defined Flink agg functions
* Support for flink count distinct

Phase Three: Stateful Operator Support(Join Operator)

* Support Regular Join
* Support Window Join

Supporting Flink in Auron is a massive and complex undertaking. We welcome community members to collaborate and discuss together.