feat: add HudiFileGroupReader with consolidated APIs to read records

[xushiyan]

Description

• Add FileGroupReader with APIs to read file slices
• Migrate existing APIs to use FileGroupReader

How are the changes test-covered

• N/A
• Automated tests (unit and/or integration tests)
• Manual tests
  • Details are described below

[codecov]

Codecov Report

Attention: Patch coverage is 91.30435% with 2 lines in your changes missing coverage. Please review.

Project coverage is 89.81%. Comparing base (2ef9557) to head (72ca4d7).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/core/src/file_group/reader.rs	92.85%	1 Missing ⚠️
crates/core/src/table/mod.rs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #164      +/-   ##
==========================================
+ Coverage   89.16%   89.81%   +0.65%     
==========================================
  Files          16       17       +1     
  Lines         877      884       +7     
==========================================
+ Hits          782      794      +12     
+ Misses         95       90       -5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.