feat: implement add-on manager #435
                
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The Problem/Issue/Bug:
No add-on manager interface.
How this PR Solves the Problem:
Add a new add-on manager interface.
In the menu:

Add dialogue:

Remove dialogue:

Manual Testing Instructions:
Install ddev-intellij-plugin-0.0.1-dev.zip
Open any DDEV project
Try to add and remove add-ons via the new interface
Related Issue Link(s):