feat: Implement an unordered scanner for append mode

[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 a new scanner for append only regions.

• Defines a ScanInput struct to keep fields required to build a scanner
• Adds a new scanner UnorderedScan to scan all sources
  • The output ordering of UnorderedScan is unspecific
  • If scan_parallelism > 1, it scans sources in parallel
  • Otherwise, it scans sources one by one
• Both UnorderedScan and SeqScan use ScanInput to construct sources
• A region uses UnorderedScan for query if it is append only (append_mode = true)
  • It uses SeqScan if append_mode = false
• Compaction still uses SeqScan

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.

[fengjiachun]

LGTM

[codecov]

Codecov Report

Attention: Patch coverage is 96.63212% with 13 lines in your changes are missing coverage. Please review.

Project coverage is 85.06%. Comparing base (14267c2) to head (60902c7).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3598      +/-   ##
==========================================
- Coverage   85.40%   85.06%   -0.34%     
==========================================
  Files         931      932       +1     
  Lines      154826   155045     +219     
==========================================
- Hits       132224   131894     -330     
- Misses      22602    23151     +549     

[v0y4g3r]

LGTM