feat: Configurable dashboard widgets

[tvdeyen]

Needs #3915 and #3916

What is this pull request for?

Allows to configure dashboard widgets.

Notable changes

Dashboard widgets are now configured in Alchemy.config

Alchemy.configure do |config|
  config.dashboard.tap do |dashboard|
    dashboard.widgets << Alchemy::Admin::Dashboard::Widget.new(id: "MyWidget", loading: "lazy")
    dashboard.stats << Alchemy::Admin::Dashboard::Widget.new(id: "MyStatistic", style: "stat")
  end
end

Checklist

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message
• I have added tests to cover this change

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.15%. Comparing base (075cc66) to head (da62148).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3917   +/-   ##
=======================================
  Coverage   98.15%   98.15%           
=======================================
  Files         343      344    +1     
  Lines        8874     8885   +11     
=======================================
+ Hits         8710     8721   +11     
  Misses        164      164           

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