feat(mito): Skeleton for scanning a region

[evenyag]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

This PR implements a skeleton for scanning a mito2 region. The mito engine creates a ScanRegion to handle a ScanRequest.

A ScanRegion collects SSTs and memtables to scan without actually reading them. It creates a Scanner to actually scan these targets in Scanner::scan(). It may use different scan modes for different scan requests so the Scanner is an enum. Now we only implement a sequential scan mode SeqScan.

classDiagram
class ScanRegion {
    -VersionRef version
    -ScanRequest request
    ~scanner() Scanner
    ~seq_scan() SeqScan
}
class Scanner {
    <<enumeration>>
    SeqScan
    +scan() SendableRecordBatchStream
}
class SeqScan {
    -ProjectionMapper mapper
    -Option~TimeRange~ time_range
    -Option~Predicate~ predicate
    -Vec~MemtableRef~ memtables
    -Vec~FileHandle~ files
    +build() SendableRecordBatchStream
}
class ProjectionMapper {
    ~output_schema() SchemaRef
    ~convert(Batch) RecordBatch
}
ScanRegion -- Scanner
ScanRegion o-- ScanRequest
Scanner o-- SeqScan
Scanner -- SendableRecordBatchStream
SeqScan o-- ProjectionMapper
SeqScan -- SendableRecordBatchStream

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Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.

Refer to a related PR or issue link (optional)

• Tracking issue for implementing mito as a region engine #1869

[codecov]

Codecov Report

Merging #2230 (8d8ccb9) into develop (8d446ed) will decrease coverage by 0.38%.
Report is 6 commits behind head on develop.
The diff coverage is 86.64%.

@@             Coverage Diff             @@
##           develop    #2230      +/-   ##
===========================================
- Coverage    85.22%   84.85%   -0.38%     
===========================================
  Files          705      709       +4     
  Lines       115706   116304     +598     
===========================================
+ Hits         98613    98687      +74     
- Misses       17093    17617     +524     

[v0y4g3r]

Mostly LGTM