[iceberg] Improved ByteBuffer string conversion for Iceberg manifests

[0dunay0]

Purpose

Paimon's Iceberg compatibility tables partitioned by a string field doesn't return any results when queried by an equality predicate on the partition field.

• The snapshot file's partition summary lower and upper bounds have a null character suffix.
• The column stats in the manifest files also contain null character suffix.

The effect of these is clients skip all the data files for the Iceberg table during scan planning. See the linked issue for examples.

Linked issue: #5007

Tests

• Added a new test case in IcebergCompatibilityTest to verify the snapshot metadata files no longer contain the \u000 padding.
• Added extensive test cases for the IcebergConversions CHAR/VARCHAR case.

API and Format

N/A

Documentation

N/A

[JingsongLi]

Thanks @0dunay0 , looks good to me!