Bundle optuna_dashboard into jupyterlab extension's package

[c-bata]

Contributor License Agreement

This repository (optuna-dashboard) and Goptuna share common code.
This pull request may therefore be ported to Goptuna.
Make sure that you understand the consequences concerning licenses and check the box below if you accept the term before creating this pull request.

• I agree this patch may be ported to Goptuna by other Goptuna contributors.

Reference Issues/PRs

None

What does this implement/fix? Explain your changes.

Currently, Jupyter Lab extension depends on the optuna-dashboard package. So we cannot introduce breaking changes on JSON APIs. This PR bundles the source code of optuna_dashboard package into the JupyterLab extension's distribution package.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.16%. Comparing base (a0a7191) to head (9067408).
Report is 12 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1037   +/-   ##
=======================================
  Coverage   70.16%   70.16%           
=======================================
  Files          35       35           
  Lines        2400     2400           
=======================================
  Hits         1684     1684           
  Misses        716      716           

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