chore: merge source_{reader,acker} impl into one

[vigith]

This is a prelude before an attempt to write a generic Trait for Source. Only once we have a Trait, we can implement other sources.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.96970% with 2 lines in your changes missing coverage. Please review.

Project coverage is 64.47%. Comparing base (7402596) to head (68b7c0b). Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
rust/numaflow-core/src/source/user_defined.rs	95.91%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2102 +/- ## ========================================== + Coverage 64.33% 64.47% +0.13% ========================================== Files 324 324 Lines 30528 30502 -26 ========================================== + Hits 19641 19666 +25 + Misses 9856 9805 -51 Partials 1031 1031 ```

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