Plugins: Never disable add new data source for core plugins

[oshirohugo]

What is this feature?

Never disable "Add new data source" for core plugin in cloud

Why do we need this feature?

This feature is needed to fix the issue where the button "Add new data source" is disabled for cloud plugins

Who is this feature for?

Plugins users

Which issue(s) does this PR fix?:

Fixes #81766

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[andresmgot]

I see that isFullyInstalled is used in another place (usePluginConfig). Wouldn't it be more correct to say that core plugins are fully installed?

[andresmgot]

Can you also add a test? :)

[andresmgot]

awesome, LGTM!