[Variant] Allow configurable JSON format for specialized types

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**

Currently, the JSON read/write methods enforce a fixed mapping for specialized variant types. For instance, timestamps are always serialized as JSON strings.

**Describe the solution you'd like**

To make this more flexible, it should be possible to pass an optional format configuration that allows users to control how specialized types (like timestamps or UUIDs) are serialized/deserialized.

**Additional context**

This feature is motivated by [datafusion-variant](https://github.com/datafusion-contrib/datafusion-variant), a higher-level library that integrates with Datafusion through a series of udfs

Start by tracing the JSON read/write methods for specialized variant types and review the datafusion-variant context linked in the issue. Define how an optional format configuration should control serialization and deserialization for timestamps and UUIDs, then verify that both directions honor the chosen formats.

Written by the indexing model from the issue text.