Rethink how SDKs are exposed to Users

[vigith]

The SDK interface can be confusing to users because not everything we have exposed can be manipulated, and even if the compile time check passes, the runtime may not.

Current Usability problems

• Numaflow User
  • I moved my Map function from UDF to Source (and vice-versa), and it is not working.
  • I have more than one map function; even if I register, only one takes effect.
  • I am able to register Map and Reduce but a vertex can be only Map or Reduce, not both.
  • Attaching a wrong container type to a vertex will not throw errors or warnings.
• Numaflow SDK Developer
  • Map DatumResponse has watermark and event-time details that are not used (should we then remove it?)

Tasks

Beta Give feedback

1. SDK signatures are confusing as it passes compile time but will not work es expected at the runtime #906
   area/sdk +1
2. remove Eventtime and Watermark from DatumReponse for Map and Reduce handlers #873
   area/sdk enhancement +2
   
3. deprecate handlerDatumMetadata #867
   area/forward area/sdk bug +3
   
4. UDF health check will pass if a map UDF is passed to a reduce vertex #908
   area/sdk +1
   
5. Go SDK Changes numaflow-go#74
   +0
6. Python SDK Changes numaflow-python#103
   +0
   
7. Java SDK Changes numaflow-java#66
   +0
   
8. Rust SDK Changes numaflow-rs#7
   +0