ARROW-11894: [Rust][DataFusion] Change flight server example to use DataFrame API

[Dandandan]

Before this PR, it uses the logical plan / optimization / physical plan more directly.
As it is meant as example, I think it's better to use the more higher level sql and collect functions.

Also, some misc cleanup.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-11894

[alamb]

BTW the test workspace check is also failing on master, so it may not be related to this PR. See more details on https://issues.apache.org/jira/browse/ARROW-11896

[alamb]

FYI I merged #9653 / ARROW-11896 for the Rust CI checks which may affect this PR. If you see "Rust / AMD64 Debian 10 Rust stable test workspace" failing with a linker error or no logs, rebasing against master will hopefully fix the problem

[alamb]

I think this is a nice cleanup @Dandandan thank you. I don't think this code is tested as part of CI link so I would like someone to test it locally before merging -- I'll do so either later today or tomorrow if no one else gets around to it

[codecov-io]

Codecov Report

Merging #9645 (b86061b) into master (bfa99d9) will decrease coverage by 0.06%.
The diff coverage is 0.00%.

@@ Coverage Diff @@## master #9645 +/- ##
==========================================
- Coverage 82.49% 82.43% -0.07% 
==========================================
Files 245 245 Lines 57347 57643 +296 ==========================================
+ Hits 47311 47517 +206 - Misses 10036 10126 +90 

Impacted Files	Coverage Δ
rust/datafusion/examples/flight_server.rs	0.00% <0.00%> (ø)
rust/parquet/src/basic.rs	87.22% <0.00%> (-10.04%)	⬇️
rust/parquet/src/arrow/array_reader.rs	77.53% <0.00%> (-0.08%)	⬇️
rust/parquet/src/schema/parser.rs	90.20% <0.00%> (ø)
rust/parquet/src/file/footer.rs	96.26% <0.00%> (+0.03%)	⬆️
rust/parquet/src/file/writer.rs	95.32% <0.00%> (+0.21%)	⬆️
rust/parquet/src/schema/types.rs	89.79% <0.00%> (+0.25%)	⬆️
rust/parquet/src/file/metadata.rs	92.96% <0.00%> (+0.74%)	⬆️
rust/arrow/src/array/equal/utils.rs	76.47% <0.00%> (+0.98%)	⬆️

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[alamb]

I tried to retrigger the jobs on this PR as I think integration is failing due to https://issues.apache.org/jira/browse/ARROW-11908

[alamb]

❤️ -- looks great @Dandandan