ARROW-17131: [Python] add StructType().field(): returns a field by name or index

[anjakefala]

[github-actions[bot]]

https://issues.apache.org/jira/browse/ARROW-17131

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[pitrou]

@anjakefala Can you take a look at the CI failures?

[anjakefala]

@pitrou Done!

[anjakefala]

Added the [] test case back =) https://github.com/apache/arrow/compare/231f2b3c4d93c0bd3e61288165d1847f83142bdb..add10bdc632f7a064120489fe37f3d950cd4683b

[ursabot]

Benchmark runs are scheduled for baseline = fa33ca400759d4e9912b06df6680bf0199e28fd5 and contender = 8c17925a63fb1d79401c50c82eefb995996b2179. 8c17925a63fb1d79401c50c82eefb995996b2179 is a master commit associated with this PR. Results will be available as each benchmark for each run completes. Conbench compare runs links: [Finished :arrow_down:0.0% :arrow_up:0.0%] ec2-t3-xlarge-us-east-2 [Failed :arrow_down:0.68% :arrow_up:0.37%] test-mac-arm [Failed :arrow_down:0.27% :arrow_up:0.27%] ursa-i9-9960x [Finished :arrow_down:0.57% :arrow_up:0.04%] ursa-thinkcentre-m75q Buildkite builds: [Finished] 8c17925a ec2-t3-xlarge-us-east-2 [Failed] 8c17925a test-mac-arm [Failed] 8c17925a ursa-i9-9960x [Finished] 8c17925a ursa-thinkcentre-m75q [Finished] fa33ca40 ec2-t3-xlarge-us-east-2 [Finished] fa33ca40 test-mac-arm [Failed] fa33ca40 ursa-i9-9960x [Finished] fa33ca40 ursa-thinkcentre-m75q 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