Make MapFieldNames public (#2118)

[tustvold]

Which issue does this PR close?

Closes #2118

Rationale for this change

See ticket

What changes are included in this PR?

Make MapFieldNames public so MapBuilder can be used

Are there any user-facing changes?

No

[alamb]

If this is a regression since 18.0.0 I think we should merge it and cut a new release candidate for 19.0.0 (and avoid releasing the regression)

[ursabot]

Benchmark runs are scheduled for baseline = 34f58f9 and contender = add2649. add2649 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