Add UDF (AKA scalar functions) support

[vhiairrassary]

Here is a PoC adding support for Ruby UDFs (AKA known as scalar functions, referenced in the issue #751). It is highly inspired from the official C API example.

The API is inspired from the Python API

DuckDB::Database.open do |db|
  db.connect do |con|
    con.register_scalar_function(
      "my_func", return_type: :integer, parameter_types: [:integer, :integer]
    ) do |x1, x2|
      (x1 + x1) * x2
    end

    result = con.query("SELECT my_func(2, 5)")
    assert_equal(result.column_count, 1)
    assert_equal(result.to_a, [[20]])
  end
end

More basic examples are available in test/duckdb_test/scalar_functions_test.rb.

The code change is more a WIP than anything else, but if you are interested about it then I would be happy to make it production ready (and if you are OK with the general structure of the change). Feedback is more than welcome.

Remaining tasks:

• Support returning nil
• Remove dynamic allocations in C function handler by fixing a maximum number of parameters
• Support volatile functions
• Types support
  • TEXT
  • INTEGER
  • DATE
  • BOOL
  • UUID
  • ARRAY
  • STRUCT
  • MAP
• Resolve remaining TODOs

EDIT: It seems there are some segfaults in the CI, I have tested it only on macOS so far. It seems to happen only when there is an exception raised in the UDF. It can be reproduced with a IRB terminal (with ./bin/console), with LLDB attached to it using the following script:

(0..1000).each do |n|
  DuckDB::Database.open do |db|
    db.connect do |con|
      con.register_scalar_function("my_func", return_type: :integer) do
        raise StandardError, 'BOOM'
      end

      begin
        result = con.query('SELECT my_func()')
      rescue
        nil
      end

      puts "#{Time.now} - #{n} - #{result.to_a}"
    end
  end
end

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[vhiairrassary]

A new PR has been merged to implement scalar functions (#970), so let's close this one