struct UnionBuilder will create child buffers with capacity

[kastolars]

Which issue does this PR close?

Closes #2523.

Rationale for this change

What changes are included in this PR?

Are there any user-facing changes?

[tustvold]

Cargo fmt wants a comma...

[kastolars]

Cargo fmt wants a comma...

Okay I added the commas. I'm not sure if there's a tool or intellij plugin where I can auto-format per those standards but I'll look it up.

[tustvold]

If you are using the Intellij-rust plugin, if you go to settings you can set it to use rustfmt instead of the built-in formatter.

You can also just run cargo fmt -all before pushing

[ursabot]

Benchmark runs are scheduled for baseline = d36f072 and contender = 87290ff. 87290ff is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java