Implement JsonSerde for all datatypes

- Fix typo in FIELD_NAME_SCALE constant
- Implement JsonSerde for DataField
- Complete serialize_json and deserialize_json for all DataType variants
- Add comprehensive test for DataType json serialization/deserialization

Resolves issue #39

Amp-Thread-ID: https://ampcode.com/threads/T-12d3046f-c788-4b3e-b1ae-6830b05aa926
Co-authored-by: Amp <[email protected]>

Update crates/fluss/src/metadata/json_serde.rs

Co-authored-by: Copilot <[email protected]>

Update crates/fluss/src/metadata/json_serde.rs

Co-authored-by: Copilot <[email protected]>

Update crates/fluss/src/metadata/json_serde.rs

Co-authored-by: Copilot <[email protected]>

Update crates/fluss/src/metadata/json_serde.rs

Co-authored-by: Copilot <[email protected]>

Author: pavlospt