feat: data buffer and related structs

[v0y4g3r]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

Add impl for the data buffer related structs for MergeTreeMemtable.

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

Comparison is base (43fd87e) 85.23% compared to head (6e778fa) 84.87%.
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3329      +/-   ##
==========================================
- Coverage   85.23%   84.87%   -0.36%     
==========================================
  Files         878      881       +3     
  Lines      143009   143843     +834     
==========================================
+ Hits       121896   122093     +197     
- Misses      21113    21750     +637     

[fengjiachun]

Almost LGTM