[elixir] Allow setting custom properties when creating a database

[nicolazar]

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Description

Currently in bindings/elixir/lib/fluss/admin.ex when creating a database, the database descriptor is not passed through, meaning that it is not possible to set comment or custom_properties on a database. The bindings hardcode None (source). The upstream Rust API supports it; the Elixir layer just doesn't expose it.

We should allow clients to set custom_properties and comment when creating a database.

Additionally, the properties should be validated to only include the values accepted by the Fluss server.

Willingness to contribute

• I'm willing to submit a PR!