Adding promotion for UnknownType per V3+ spec

[ldsantos0911]

Rationale for this change

When attempting to write a PyArrow dataframe that has a null field corresponding to a nullable Table field, I see this error:

ValueError: Mismatch in fields:
┏━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━━━┓
┃    ┃ Table field              ┃ Dataframe field           ┃
┡━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━━━┩
│ ❌ │ 1: col1: optional string │ 1: col1: optional unknown │
│ ✅ │ 2: col2: optional string │ 2: col2: optional string  │
└────┴──────────────────────────┴───────────────────────────┘

Per the V3 spec, UnknownType (as pa.null() is cast) should be promotable to any type. This change attempts to unblock situations where a DataFrame may end up having a null type for an optional field while also incorporating the V3 spec.

Note: This issue is related but was based on an older version. Nonetheless, the underlying situation is still blocked.

Are these changes tested?

Yes. Added a couple of relevant unit tests. Additionally, I did a manual sanity check:

Python 3.10.18 (main, Jun  4 2025, 08:06:20) [Clang 16.0.0 (clang-1600.0.26.3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import pyarrow as pa
>>> from pyiceberg.catalog import load_catalog
>>> from pyiceberg.schema import Schema
>>> from pyiceberg.types import StringType, NestedField
>>> warehouse_path = '/tmp/warehouse'
>>> catalog = load_catalog('default', **{'type': 'sql', 'uri': f"sqlite:///{warehouse_path}/pyiceberg_catalog.db", 'warehouse': f"file://{warehouse_path}"})
>>> table = catalog.load_table('default.test2')
>>> table.schema()
Schema(NestedField(field_id=1, name='col1', field_type=StringType(), required=False), NestedField(field_id=2, name='col2', field_type=StringType(), required=False), schema_id=0, identifier_field_ids=[])
>>> df = pa.Table.from_pylist([{'col1': None, 'col2': 'blah'}])
>>> df.schema
col1: null
col2: string
>>> table.append(df)
>>> x = table.scan().to_arrow()
>>> x
pyarrow.Table
col1: large_string
col2: string
----
col1: [[null]]
col2: [["blah"]]

Are there any user-facing changes?

Yes, this will allow UnknownType promotion.