feat: support for user-defined source

[yhl25]

• support for user-defined source
• refactor mapstream to avoid exposing gRPC stream observer
• code formatting

Fixes https://github.com/numaproj/numaflow/issues/931

[KeranYang]

@yhl25 This single PR could be separated into 3(mapstream refactor, udsource and reformatting). It's in general best practice to make small changes one at a time and make each PR dedicated to a single feature. The benefits top of my head are 1. it is easy to review as the reviewer doesn't need to context switch during a review, 2. easy to track the changes using the commit log, which leads to 3. easy to rollback a single logical change for debugging purposes without reverting other features/refactors, etc.

[vigith]

please cut a release.

[KeranYang]

@yhl25 I wrote an E2E test on the platform side to verify this change. Looking good. If no concerns, could you cut a new release? Thanks!