AVRO-4351: [C++] Preserve primitive custom attributes

[wgtmac]

What is the purpose of the change

Avro C++ currently drops custom attributes attached directly to primitive
schema objects.

The schema compiler does not collect unknown properties from primitive objects,
and NodePrimitive::printJson only emits an object for logical types without
serializing primitive custom attributes.

This is required by integrations such as Apache Iceberg. Iceberg uses the
adjust-to-utc property to distinguish timestamp from timestamptz, while
both types use Avro long with the timestamp-micros logical type.

This change:

• Preserves unknown properties when parsing primitive schema objects.
• Serializes a primitive as a JSON object when it has a logical type or custom
  attributes.
• Keeps the implementation generic and does not add Iceberg-specific behavior.

Verifying this change

This change added tests and can be verified as follows:

• Added a schema round-trip test for Iceberg's adjust-to-utc property.
• Added coverage for a generic primitive custom property without a logical type.

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

[wgtmac]

@martin-g @thiru-mg @Fokko Could you help review this? Thanks!