[python] Fix GenericVariant UUID decoding for Java compatibility and support datetime/UUID

[juntaozhang]

Purpose

• This PR extends pypaimon's GenericVariant to support encoding native Python datetime and uuid.UUID values, and fixes the UUID binary decoding byte order to align with Java.
• Completes the Python API documentation for supported shredding schema types

Tests

• Unit tests: paimon-python/pypaimon/tests/variant_test.py
• Java-Python E2E tests: JavaPyE2ETest and java_py_read_write_test.py

[JingsongLi]

• Python datetime encoding relies on a floating-point calculation (total_seconds() * 1_000_000), which leads to a loss of microsecond precision for dates far removed from the epoch.
• Empirical testing shows a round-trip for 9998-07-08 ...654321 results in ...654336 (an error of 15μs); errors of ±1μs also occur for the years 1600, 1900, 2500, and 5000.
• The implementation should be changed to use pure integer arithmetic based on days, seconds, and microseconds.

[juntaozhang]

• Python datetime encoding relies on a floating-point calculation (total_seconds() * 1_000_000), which leads to a loss of microsecond precision for dates far removed from the epoch.

  • Empirical testing shows a round-trip for 9998-07-08 ...654321 results in ...654336 (an error of 15μs); errors of ±1μs also occur for the years 1600, 1900, 2500, and 5000.

  • The implementation should be changed to use pure integer arithmetic based on days, seconds, and microseconds.

@JingsongLi Thanks for your review. I have fixed the issue, PTAL.