[Buffered Reader] Implement PrefetchBlock by extending Existing Block

[raj-prince]

Description

• Create PrefetchBlock by Extending the Existing Block

Link to the issue in case of a bug fix.

b/426061112

Testing details

1. Manual - NA
2. Unit tests - NA
3. Integration tests - NA

Any backward incompatible change? If so, please explain.

[codecov]

Codecov Report

Attention: Patch coverage is 82.75862% with 5 lines in your changes missing coverage. Please review.

Project coverage is 78.85%. Comparing base (07a1ee6) to head (80ad00b).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/block/prefetch_block.go	81.48%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3432      +/-   ##
==========================================
- Coverage   78.85%   78.85%   -0.01%     
==========================================
  Files         140      141       +1     
  Lines       18472    18499      +27     
==========================================
+ Hits        14566    14587      +21     
- Misses       3421     3427       +6     
  Partials      485      485              

Flag	Coverage Δ
unittests	78.85% <82.75%> (-0.01%)	⬇️

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