[elixir] Add support for complex types - array, map & row

[nicolazar]

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• I searched in the issues and found nothing similar.

Description

The Elixir binding's data-type representation is flat (scalars + decimal/char/binary), with no support for nested Array / Map / Row types. Core already supports them (recursive DataType, nested row format; Python added this in #601).

Two layers to cover:

• Schema representation: add recursive variants to the Nif DataType enum and nest the Elixir type ({:array, dt}, {:map, k, v}, {:row, [{name, dt}]}); make to_fluss_type/from_fluss_type recursive.
• Data plane: extend the row converters (field_to_term/set_field_from_term)
  to read/write nested values.

Willingness to contribute

• I'm willing to submit a PR!