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

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.

Closes #9645 from Dandandan/flight_server_dataframe_example

Authored-by: Heres, Daniel <61924dac3230581b78b62e9e0cd0f8203281db27@gmail.com>
Signed-off-by: Andrew Lamb <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@nerdnetworks.org>

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