Add serialization of `ScalarValue::Struct`

[alamb]

Draft as it builds on https://github.com/apache/arrow-datafusion/pull/3535

Which issue does this PR close?

Closes https://github.com/apache/arrow-datafusion/issues/3531

Rationale for this change

See https://github.com/apache/arrow-datafusion/issues/3531

What changes are included in this PR?

Changes:

1. Implement serialization code for: ScalarValue::Struct
2. Add tests

Are there any user-facing changes?

No