feat: Implement KeyDictBuilder for the merge tree memtable

[evenyag]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

This PR implements the KeyDictBuilder for the merge tree memtable

• Each builder has fixed capacity
• It interns key and returns an index to get the key from the builder again
• It can scan keys in sorted order from the builder
• It can build a KeyDict by invoking finish()
  • It will replace pk index with a new value so the pk index has the same order the key

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

Comparison is base (43fd87e) 85.23% compared to head (3d8da26) 84.82%.
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3334      +/-   ##
==========================================
- Coverage   85.23%   84.82%   -0.41%     
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  Files         878      882       +4     
  Lines      143009   144085    +1076     
==========================================
+ Hits       121896   122225     +329     
- Misses      21113    21860     +747     

[fengjiachun]

LGTM

[v0y4g3r]

LGTM