feat: Support for on success sink

[vaibhavtiwari33]

Closes #206

As part of #3043, we're adding support for OnSuccess sink which allows users to write to a separate sink when writing to primary sink succeeds for example.
This PR aims to add this support to Java SDK as well, giving the users ability to interact with the onSuccess sink.

Local testing:

Brought up a pipeline with a UDSink using java SDK with latest changes:

Able to see messages in the onSuccess UDSink being routed from the primary UDSink:

Mix of messages being written to fallback and onsuccess sink:

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 14 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@38734c0). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...main/java/io/numaproj/numaflow/sinker/Service.java	70.00%	1 Missing and 8 partials ⚠️
...n/java/io/numaproj/numaflow/sinker/GRPCConfig.java	0.00%	2 Missing and 1 partial ⚠️
...ain/java/io/numaproj/numaflow/sinker/Response.java	94.59%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #207   +/-   ##
=======================================
  Coverage        ?   60.78%           
  Complexity      ?      517           
=======================================
  Files           ?      152           
  Lines           ?     3448           
  Branches        ?      242           
=======================================
  Hits            ?     2096           
  Misses          ?     1181           
  Partials        ?      171           

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[vaibhavtiwari33]

The current numaflow-core implementation for accepting onSuccess response from server allows sending None in lieu of explicit OnSuccessMessage, but the generated java protobuf for OnSuccessMessage prohibits setting the OnSuccessMessage as null, so a default instance is being sent to the client currently.

This leads to discrepancy between behaviours across different SDKs.

Question: Should we add functionality to numaflow-core to interpret empty value ("") as an indication of sending original message to the onSuccess sink?

[yhl25]

Please add java doc for public classes.