[Variant] Avoiding extra splice in `ObjectBuilder::finish` if possible

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

This come up from @alamb comments from https://github.com/apache/arrow-rs/pull/7935#discussion_r2213491567

> As a follow on PR we can consider avoiding this extra splice somehow (by preallocating the size or something). Future work though

**Describe the solution you'd like**

preallocating the size or something to avoid the splice

**Describe alternatives you've considered**

**Additional context**

Start by locating the Rust implementation of ObjectBuilder::finish and read the linked discussion in apache/arrow-rs#7935 for the extra splice context. Determine whether the output size can be known early enough to avoid that splice; done means the extra allocation or splice is avoided without changing the builder's behavior, with relevant existing checks still passing.

Written by the indexing model from the issue text.