Add keyvault reference feature on apps environment variables

**Is your feature request related to a problem? Please describe.**

My dev Azure Spring Apps service is used by multiple people, mostly developers.
As this service instance is not a production one, I gave to developers more permissions, for example the ability to read and modify the environment variables on apps.
That means that every developers can see the environments variables values on every apps on this service instance.

In most cases, it is not a problem as developers should be aware of the environments variables which are given to their apps.

But, some of these variables should stay hidden, even for the developers.
A good example is the DT_TENANTTOKEN one.
As we are using dynatrace integration, we need to give this secret to the app with an environment variable, but, by doing this, the value is visible by every developer, but they are not concerned by this value.

**Describe the solution you'd like**

Adding the feature "KeyvaultReference" as it exists on app service may prevent people to see the environment value, as it may be stored in a keyvault on which they won't have access, but the app identity will !

**Describe the Customer Impact**

Either give access to environment variable to peoples and leaks some secrets they should not known,
either don't give them access to environment variable to prevent secret leaks, but it is not acceptable for us because developers needs to have access to this kind of informations on their apps