Azure System Managed Identity support

### 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.

Aspire automatically sets up user assigned managed identities and creates role assignments so that Container Apps can access the Container Registry, Storage, Log Analytics, etc.

The lifecycle of these identities is (or should be!) closely tied to the resources they're serving. This is the ideal use for system managed identities.

### Describe the solution you'd like

Instead of creating user assigned managed identities, use system managed identities by default.

### Additional context

_No response_

No files or tests are named. Start by tracing where Aspire provisions user-assigned managed identities and role assignments for Container Apps and their dependent resources; compare the requirements for Azure system-managed identities. Done means system-managed identities are used by default while required registry, storage, and Log Analytics access continues to work.

Written by the indexing model from the issue text.