GenericByteViewArray: support finding total length of all strings

[neilconway]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

It would be useful to provide a method to determine the total length of all the strings in a StringViewArray. For example, in the implementation of concat() and concat_ws() UDFs in DataFusion, this would be useful to pre-allocate the output buffers for the result of the UDF.

When the input is a StringViewArray, we currently use total_buffer_bytes_used() as a capacity hint, but this underestimates when the array contains short strings (discussion).

Describe the solution you'd like

A method on GenericByteViewArray like so:

/// Returns the total byte length of all strings in the array.
pub fn total_bytes_len(&self) -> usize {
    self.views()
        .iter()
        .map(|v| (*v as u32) as usize)
        .sum()
}

Describe alternatives you've considered

total_buffer_bytes_used() is better than nothing but it underestimates.

We could iterate over the views and take the lowest 32-bits ourselves, but depending on Arrow implementation details like that seems unfortunate.

Additional context