Add nulls_equal option to DataFrame.join/3

[ntodd]

Adds support for Polars' nulls_equal join parameter to DataFrame.join/3, allowing NULL values to match during join operations. Defaults to false (standard SQL semantics where NULL != NULL).

Changes

• Added :nulls_equal option to DataFrame.join/3 public API
• Refactored NIF boundary to use an options map (ExJoinOptions struct with #[derive(NifMap)]) to make other options easier to add in the future
• Added tests for all join types with nulls_equal: true

Example

left = Explorer.DataFrame.new(a: [1, nil], b: ["a", "b"])
right = Explorer.DataFrame.new(a: [1, nil], c: ["d", "e"])

# Default behavior (SQL semantics): nil != nil
Explorer.DataFrame.join(left, right)
# => 1 row (only matches on a=1)

# With nulls_equal: true
Explorer.DataFrame.join(left, right, nulls_equal: true)
# => 2 rows (matches on a=1 and a=nil)