Re-export NullBufferBuilder in the arrow crate

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
I thought there was a clever optimization for handling data with no nulls which I filed in arrow-rs

• Optimize BooleanBufferBuilder for non nullable columns #6973

However, @tustvold pointed out that NullBufferBuilder has exactly the optimization described

I wondered how I could have missed this before and then I realized that the reason is likely that NullBufferBuilder is not re-exported in the arrow crate

You can see this here:
https://docs.rs/arrow/latest/arrow/index.html?search=NullBufferBuilder

NullBuffer is exported (as arrow::buffer::NullBuffer) however:

You can use NullBufferBuilder only if you explicitly bring in the arrow_buffer crate:
https://docs.rs/arrow-buffer/latest/arrow_buffer/builder/struct.NullBufferBuilder.html

Describe the solution you'd like
Export NullBufferBuilder to be consistent with other structures that are reexported (like arrow::array::BooleanBufferBuilder https://docs.rs/arrow/latest/arrow/array/struct.BooleanBufferBuilder.html)

Describe alternatives you've considered

Additional context