Dealing with broken configuration references

[christophenne]

Is your feature request related to a problem? Please describe.
A user can enter different kinds of references to choose config values for a package (e.g. ConfigMap, Secret, Package Config), both on the UI and CLI. They don't yet see whether the entered reference (e.g. namespace + name + key in the case of a ConfigMap) can be resolved and is valid (at least on the UI).

Describe the solution you'd like
For a reference input, the user should see some hint of whether the entered reference is:

• existing and passes validation
• existing but does not pass validation
• not existing

To be discussed: This kind of hint should probably not be a strict validation? As also described in #495, when a referenced object gets changed or deleted, the configuration could become invalid. We need a way to deal with this.

There are lots of ways in which a reference could become invalid:

• a referred value changes, and the new value is not valid according to the glasskube package anymore
• a referred value is deleted (e.g. because the ConfigMap, Secret or Package has been removed)

Describe alternatives you've considered

Additional context
Follow-Up of #121
Related to #495

[kosmoz]

Notably, the operator already deals with this by not touching any dependent resources and setting a failed condition on the Package resource. This ticket is only about how to handle this situation on the UI (and to a lesser extend CLI).