Add example of middleware with async usage

[AngelicosPhosphoros]

I think, this is useful because allows user to avoid usage of futures_util.

I don't know if I must adjust existing introduction or add new example project to the middleware folder so I added it to existing example for now.

I can change it if needed.

[codecov]

Codecov Report

Merging #577 (3fecef2) into master (7efda5e) will increase coverage by 0.01%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master     #577      +/-   ##
==========================================
+ Coverage   85.82%   85.84%   +0.01%     
==========================================
  Files         111      112       +1     
  Lines        5722     5770      +48     
==========================================
+ Hits         4911     4953      +42     
- Misses        811      817       +6     

Impacted Files	Coverage Δ
examples/middleware/introduction_await/src/main.rs	87.50% <87.50%> (ø)

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[msrd0]

Hi, thanks for the PR!

I think it makes more sense to add a second example (say introduction_await) that does the same as the current example, just with using await instead of futures-util's function calls.

[AngelicosPhosphoros]

I added new example project and removed changes from old example.

[msrd0]

Thanks