Stream features to offline storage

[hangfei]

We now support stream features to online table(Redis).

It's a popular ask by users.

The main code is added by this #137.
Existing user guide: https://github.com/linkedin/feathr/blob/f3d6ba57425d550f19730403115c0523f6c3ffe6/docs/how-to-guides/streaming-source-ingestion.md.

Difficulty: medium

[hangfei]

@ahlag If you are interested, feel free to assign this to yourself.

[ahlag]

@hangfei
Yeah sure! By the way, I can't assign myself. I think it's because I don't have the permission. By the way, how does this differ from this issue #289?

[hangfei]

#289 provide an API to "query" offline data.. This provides an API to write streaming data to offline.

[gkorland]

Did you consider having Redis Streams as a lightweight alternative Stream to Kafka.
Notice Redis already has a nice integration with Spark Streaming