[elixir] Allow setting custom properties when creating a table

[nicolazar]

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Description

Similarly to #587, it is not currently possible to set custom properties on a table from the elixir bindings.

The existing :properties option routes to the core .property() builder, which validates against known Fluss table properties and rejects arbitrary keys. For example, properties: [{"k", "v"}] fails with:

invalid_config_exception (code 39): 'k' is not a Fluss table property. Please use '.customProperty(..)' to set custom properties.

The upstream Rust TableDescriptor builder already supports this — .comment(...), .custom_property(...), and .custom_properties(...) in crates/fluss/src/metadata/table.rs. The Elixir layer simply doesn't expose them.

Willingness to contribute

• I'm willing to submit a PR!