feat: Metadata propagation in source, source transformer, mapper, sink

[vaibhavtiwari33]

Fixes: #205

Notes

Change in OnSuccess message metadata

With the introduction of a shared UserMetadata class, the userMetadata member in Message class for OnSuccess message has been changed from:

   private final HashMap<String, KeyValueGroup> userMetadata;

to

    private final UserMetadata userMetadata;

This is done so that the users can utilize the same UserMetadata for onSuccess message without conversion.

Furthermore, the new UserMetadata has been implemented in its current form since it allows parity with other SDKs (inner data holds Map of Map, instead of Map of KeyValueGroup) and also prevents the extra step of converting proto KeyValueGroup object to local KeyValueGroup class object and back.

Format of userMetadata and systemMetadata

Both UserMetadata and SystemMetadata classes have inner data in the format:

Map<String, Map<String, byte[]>> data;

[codecov]

Codecov Report

❌ Patch coverage is 72.00000% with 49 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@3180f88). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...main/java/io/numaproj/numaflow/sinker/Message.java	20.68%	22 Missing and 1 partial ⚠️
...ain/java/io/numaproj/numaflow/sourcer/Message.java	23.07%	9 Missing and 1 partial ⚠️
...o/numaproj/numaflow/sourcetransformer/Message.java	28.57%	4 Missing and 1 partial ⚠️
...java/io/numaproj/numaflow/shared/UserMetadata.java	94.20%	0 Missing and 4 partials ⚠️
...va/io/numaproj/numaflow/shared/SystemMetadata.java	92.30%	1 Missing and 1 partial ⚠️
...java/io/numaproj/numaflow/mapper/HandlerDatum.java	50.00%	1 Missing ⚠️
.../java/io/numaproj/numaflow/mapper/MapperActor.java	80.00%	0 Missing and 1 partial ⚠️
.../numaproj/numaflow/sourcer/OutputObserverImpl.java	0.00%	0 Missing and 1 partial ⚠️
...aproj/numaflow/sourcetransformer/HandlerDatum.java	50.00%	1 Missing ⚠️
...j/numaflow/sourcetransformer/TransformerActor.java	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #210   +/-   ##
=======================================
  Coverage        ?   60.96%           
  Complexity      ?      560           
=======================================
  Files           ?      155           
  Lines           ?     3571           
  Branches        ?      252           
=======================================
  Hits            ?     2177           
  Misses          ?     1215           
  Partials        ?      179           

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[yhl25]

• Use function overloading and introduce a method for constructing on success response using the Datum.
• Remove onSuccess function for proto and use a private method.
• Add an example(could be a follow-up PR).