Memory accounting in GenericBytesView overcounts allocation size

[alamb]

          This is something I also noticed in #7082

One thing to note is that NullBufferBuilder::allocated_size() is used here:

arrow-rs/arrow-array/src/builder/generic_bytes_view_builder.rs

Lines 397 to 408 in 92cfd99

	/// Return the allocated size of this builder in bytes, useful for memory accounting.
	pub fn allocated_size(&self) -> usize {
	let views = self.views_builder.capacity() * std::mem::size_of::<u128>();
	let null = self.null_buffer_builder.allocated_size();
	let buffer_size = self.completed.iter().map(|b| b.capacity()).sum::<usize>();
	let in_progress = self.in_progress.capacity();
	let tracker = match &self.string_tracker {
	Some((ht, _)) => ht.capacity() * std::mem::size_of::<usize>(),
	None => 0,
	};
	buffer_size + in_progress + tracker + views + null
	}

And I think it's used with assumption it provides bytes not bits, so may need adjustment.

Originally posted by @Jefffrey in #7089 (review)

I think the adjustment is to divide the allocated_size by 8.

Another solution would be to deprecate allocated_size entirely and make a new function that returns allocated size in bytes