feat: metadata implementation for source, map and sink

[BulkBeing]

numaproj/numaflow#2811

Tested by modifying the existing examples and running below pipeline. The simple-source generates some user metadata, then the map vertex adds additional groups to this and forwards to sink. Verified all user metadata is available at the sink vertex.

apiVersion: numaflow.numaproj.io/v1alpha1
kind: Pipeline
metadata:
  name: simple-source
spec:
  vertices:
    - name: in
      scale:
        min: 1
        max: 1
      source:
        udsource:
          container:
            # A simple user-defined async source
            image: simple-source-py:v5
            imagePullPolicy: Never
      limits:
        readBatchSize: 2
    - name: flatmap
      scale:
        min: 1
        max: 1
      udf:
        container:
          image: map-flatmap-py:v5
          imagePullPolicy: Never
          # image: quay.io/numaio/numaflow-go/map-forward-message:stable
          # imagePullPolicy: Always
          env:
            - name: PYTHONDEBUG
              value: "true"
      containerTemplate:
        env:
          - name: NUMAFLOW_DEBUG
            value: "true" # DO NOT forget the double quotes!!!
    - name: inbuilt
      sink:
        log: {}
    - name: out
      scale:
        min: 1
        max: 1
      sink:
        udsink:
          container:
            args:
            - python
            - example.py
            image: sink-log-py:v2
            imagePullPolicy: Never
            env:
              - name: PYTHONDEBUG
                value: "true"
              - name: INVOKE
                value: "func_handler"
  edges:
    - from: in
      to: flatmap
    - from: flatmap
      to: inbuilt
    - from: flatmap
      to: out

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.89%. Comparing base (2990279) to head (b0c7e28).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #274      +/-   ##
==========================================
+ Coverage   93.63%   93.89%   +0.25%     
==========================================
  Files          65       66       +1     
  Lines        2907     3013     +106     
  Branches      152      156       +4     
==========================================
+ Hits         2722     2829     +107     
+ Misses        135      134       -1     
  Partials       50       50              

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