feat: Implement append mode for a region

[evenyag]

I hereby agree to the terms of the GreptimeDB CLA.

Refer to a related PR or issue link (optional)

• Support append only table #3480

What's changed and what's your intention?

This PR implements append mode for a mito region.

• By default, append_mode is false and the engine removes duplicate rows.
• If append_mode is true, the engine keeps all duplicate rows.

It adds a dedup flag to the MergeReader. The MergeReader will keep rows with duplicate keys but still yield rows in sorted order.

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.

[codecov]

Codecov Report

Attention: Patch coverage is 94.05594% with 17 lines in your changes are missing coverage. Please review.

Project coverage is 84.84%. Comparing base (653697f) to head (e45e73b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3558      +/-   ##
==========================================
- Coverage   85.15%   84.84%   -0.31%     
==========================================
  Files         929      930       +1     
  Lines      154003   154236     +233     
==========================================
- Hits       131137   130862     -275     
- Misses      22866    23374     +508     

[fengjiachun]

LGTM

[waynexia]

How to set this mode? Rest LGTM

[evenyag]

How to set this mode? Rest LGTM

I'll expose this to table options in another PR.

[killme2008]

LGTM