Message Consumption flow high level arch

[gauravAshok]

WIP: https://docs.google.com/document/d/1dZaoYPjwutHQgUuRdJYqHV7ejAUe057TtXarn-9Crpg/edit#heading=h.8sejif1s7gf1

Enhance the above doc with high level arch of how consume flow will look like. Add control flow and data flow diagrams. Add major entities involved.
Add info on relevant module & code structure required.

Sharded Subscription HLD:
https://github.com/gauravAshok/varadhi/blob/sharding_exploration/docs/InitialShardBasedImpl.md