feat: impl merge reader for DataParts

[v0y4g3r]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

This PR implements the merge reader for DataParts that merges data bacthes from all parts inside one DataParts, and yield batches according to pk weight and timestamp.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR does not require documentation updates.

Refer to a related PR or issue link (optional)

• Tracking issue for the memtable improvement #2804

[codecov]

Codecov Report

Attention: Patch coverage is 84.46215% with 78 lines in your changes are missing coverage. Please review.

Project coverage is 85.01%. Comparing base (7c88d72) to head (d83e3fe).
Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3361      +/-   ##
==========================================
- Coverage   85.13%   85.01%   -0.13%     
==========================================
  Files         887      892       +5     
  Lines      144657   146042    +1385     
==========================================
+ Hits       123157   124160    +1003     
- Misses      21500    21882     +382     

[evenyag]

Mostly LGTM

[fengjiachun]

DLJB