[FLUSS-2123][flink] Added Flink Wrapper for Histogram Statistics

[rionmonster]

Purpose

Linked issue: close #2123

Per Issue #2123, this pull request addresses a potential null reference that could occur when histogram-related statistics were requested from Flink (as the existing statistics would be returned as explicitly null).

Brief change log

This change replaces the existing logic (i.e. returning null explicitly) and instead wraps the existing histogram statistics within an available FlinkHistogramStatistics instance to ensure the underlying values can be accessed through this interface.

Tests

These changes were initially reproduced within a unit test that was later transitioned into the new FlinkHistogramTest class which verifies the intended fix but also includes a suite of test cases to verify other histogram-related operations (e.g., accessing various statistics, etc.)

API and Format

N/A

Documentation

N/A

[rionmonster]

CC: @wuchong (as an original author for the related classes) / @spoon-lz (as the reporter)

Please let me know if you'd like any changes made -- the issue and related fix were pretty straightforward.