Deferred cesiumIonKey initialization

[FelixGirard]

Is your feature request related to a problem? Please describe.
Since we use a service (PSS) to get the key, we need to do it after we are authenticated.

Describe the solution you'd like
I would like to have a deferred way to set the cesiumIonKey.

Describe alternatives you've considered

Additional context
Seems like you embrace the deferred initialization for other properties: #6449
So I think it would make sense to apply that to others.

[ben-polinsky]

This makes sense. We'll get someone on it.

[pmconne]

Most of the context for this issue is in the issue linked by @FelixGirard, so let's discuss it there.

[tcobbs-bentley]

Based on #6526, should this issue going to be closed as not planned?