Add OptunaHub example

[nabenabe0928]

Reference Issue

No issue related.

What does this PR implement/fix? Explain your changes.

This PR adds an example of OptunaHub.

How should this PR be tested?

Please simply run the code.

Any other comments?

The following comment in the PR template is deprecated.

Please make sure that:
* you updated all docs, this includes the changelog (doc/progress.rst)

[SubhadityaMukherjee]

Looks good and works well :)
I added an option to publish results back to OpenML as that would be good to show the users.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.24%. Comparing base (a4fb848) to head (9485f50).
Report is 6 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1386      +/-   ##
===========================================
- Coverage    84.32%   80.24%   -4.09%     
===========================================
  Files           38       38              
  Lines         5315     5325      +10     
===========================================
- Hits          4482     4273     -209     
- Misses         833     1052     +219     

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