feat: Defines structs in 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 defines common structs for a new memtable, the merge tree memtable.

• The merge tree memtable divides a tree into multiple partitions.
• Each partition contains multiple shards.
• Each shard contains a key dictionary and a list of data parts.
• The tree finds a shard by a row's primary key and inserts the row into the mutable data part of the shard.
• It freezes mutable parts into immutable parts and merges them later

It also ports necessary codes to implement the Memtable trait from the time series memtable.

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

Comparison is base (29f11d7) 85.57% compared to head (73991b6) 84.77%.
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3326      +/-   ##
==========================================
- Coverage   85.57%   84.77%   -0.81%     
==========================================
  Files         872      878       +6     
  Lines      141958   142704     +746     
==========================================
- Hits       121481   120976     -505     
- Misses      20477    21728    +1251     

[fengjiachun]

LGTM

[v0y4g3r]

LGTM