Allow user to define histogram size

I'm using perfetto UI for a lot of performance debugging. I see there is an option to create histogram from the pivot table. Currently, it seems the histogram size is automatically calculated and no way to override it.

It would be very useful if there is a way to allow users to define histogram size (e.g. number of buckets, or size of the bucket) so we can get more fine grain distribution.

Start at the Perfetto UI pivot-table histogram entry point and trace how histogram size is currently calculated. Define how users should provide a bucket count or bucket size, then verify that the resulting histogram uses that setting and preserves the existing behavior when no override is provided.

Written by the indexing model from the issue text.