feat: Implement iter for the new memtable

[evenyag]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

This PR mainly implements the iter() method for the new memtable.

• The iter() method returns a TreeIter to iterate partitions
• The memtable can use filters to prune partitions
• The TreeIter read partitions one by one
• For each partition, the iter creates a PartitionReader
• The PartitionReader merges DataBatches from different ShardReaderImpls and the ShardBuilderReaders
• Each ShardReader merges DataBatches from different data parts.

Unresolved problem:

• Integrates with the DedupReader

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: Patch coverage is 76.24190% with 110 lines in your changes are missing coverage. Please review.

Project coverage is 85.21%. Comparing base (afe4633) to head (3337cfa).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3373      +/-   ##
==========================================
- Coverage   85.60%   85.21%   -0.40%     
==========================================
  Files         893      893              
  Lines      146815   147210     +395     
==========================================
- Hits       125676   125438     -238     
- Misses      21139    21772     +633     

[v0y4g3r]

LGTM

[fengjiachun]

LGTM