Add `finish_cloned` for `BufferBuilder<T: ArrowNativeType>`

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

So we can get the clone of current status of the buffer with `&T`

**Describe the solution you'd like**

**Describe alternatives you've considered**

**Additional context**

Start by locating BufferBuilder and reviewing its existing completion methods and related tests. Determine the expected cloned representation of the current buffer status, then add coverage showing that finish_cloned returns that status without consuming the builder.

Written by the indexing model from the issue text.