feat: merge tree data parts

[v0y4g3r]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

• Define DataPart and DataParts, and add read method for it.
• Rename iter method in DataBuffer to read as it no long implements Iterator trait.

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: 15 lines in your changes are missing coverage. Please review.

Comparison is base (fa08085) 85.28% compared to head (8269530) 84.87%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3346      +/-   ##
==========================================
- Coverage   85.28%   84.87%   -0.42%     
==========================================
  Files         884      884              
  Lines      144265   144401     +136     
==========================================
- Hits       123042   122565     -477     
- Misses      21223    21836     +613     

[evenyag]

LGTM

[fengjiachun]

LGTM