feat(test): a simple in-memory buffer for simulating ISB

[vigith]

Closes #3198

[codecov]

Codecov Report

❌ Patch coverage is 98.67424% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.61%. Comparing base (dbdfdbd) to head (4098879).

Files with missing lines	Patch %	Lines
rust/numaflow-testing/src/simplebuffer.rs	94.71%	12 Missing ⚠️
rust/numaflow-testing/src/simplebuffer/reader.rs	99.53%	1 Missing ⚠️
rust/numaflow-testing/src/simplebuffer/writer.rs	99.55%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3213      +/-   ##
==========================================
+ Coverage   80.26%   80.61%   +0.34%     
==========================================
  Files         297      302       +5     
  Lines       67629    68685    +1056     
==========================================
+ Hits        54283    55367    +1084     
+ Misses      12796    12765      -31     
- Partials      550      553       +3     

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[yhl25]

Can we make write method return a future and resolve it using resolve method? Also let's make sure we can inject errors for during future resolution?