Adding a check in plugin registry to check for valid registry urls/names

Description:

In settings.json when we try adding a simple registry with name test and with link https://github.com it should give an error.

After that refresh list of plugins in the current workspace and start a new workspace and try to list plugins in that new workspace. You shouldn't see plugins in the workspace because of the github.com registry. 

We would like to request for a check for valid registry inputs.