MINOR: Add datafusion-sql example

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

• In preparation for the 9.0.0 release I thought it would be useful to add an example to the new datafusion-sql crate so that it is easier for users to get started. It also documents our public API to some degree.

What changes are included in this PR?

• New example
• Update CI to run the example
• Updated README

Are there any user-facing changes?

No

[codecov-commenter]

Codecov Report

Merging #2693 (02bf992) into master (3c1c188) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2693      +/-   ##
==========================================
- Coverage   84.70%   84.70%   -0.01%     
==========================================
  Files         268      268              
  Lines       47059    47060       +1     
==========================================
  Hits        39861    39861              
- Misses       7198     7199       +1     

Impacted Files	Coverage Δ
datafusion/expr/src/logical_plan/builder.rs	91.94% <0.00%> (-0.18%)	⬇️
datafusion/expr/src/logical_plan/plan.rs	67.63% <0.00%> (-0.27%)	⬇️
datafusion/optimizer/src/utils.rs	32.79% <0.00%> (+0.53%)	⬆️

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[alamb]

Looks good to me. 👌