Allow UAMI's to be added to container apps

### Is there an existing issue for this?

- [x] I have searched the existing issues

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

I am trying do add a User Assigned Managed Identity (UAMI) to a container app so when I do: azd up - the UAMI is set as a User Identity on the Container App. (eventually there will be multiple UAMI's on the app)
This currently doesn't seem possible to do in an "Aspire" way, am I missing something?
Every AI I ask suggests something like these on the builder:
WithUserAssignedIdentity and WithUserAssignedManagedIdentity
But those methods don't exist, perhaps they used to (given multiple AI's suggest it)?
I can import existing in the bicep, but you can't modify existing resources, only modify them.
So this leaves me with creating the apps in bicep, which also means I need to create docker images etc. Doing this is fine if that is what is required, but if 'feels' like I am moving further away from "the Aspire way".
Again, am I missing something? If so, what is the Aspire way to assign n UAMI's to a container app?

### Describe the solution you'd like

a method like WithUserAssignedIdentity that I can call on the builders in the AppHost program.cs would be ideal.

### Additional context

_No response_