Fix BinaryBuilder and StringBuilder Capacity Allocation in StructBuilder

[chunshao90]

…parameters in struct_builder

Which issue does this PR close?

Closes #2783 .

Rationale for this change

What changes are included in this PR?

Fix BinaryBuilder and StringBuilder initialization parameters in struct_builder.

Are there any user-facing changes?

No.

[tustvold]

Thank you for this, looks good to me 👍

[ursabot]

Benchmark runs are scheduled for baseline = 70bcfbc and contender = 333633e. 333633e 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