Extract UserAssignedIdentity resource from AzureContainerAppEnvironmentResource and AzureAppServiceEnvironmentResource

Similar to what was done with AzureContainerRegistryResource in https://github.com/dotnet/aspire/pull/13306, we should extract the UserAssignedIdentity that is created by AzureContainerAppEnvironmentResource and AzureAppServiceEnvironmentResource into a separate AzureUserAssignedIdentityResource instance.

This way consumers can get access to the identity and add role assignments to it, if necessary.

See https://github.com/dotnet/aspire/pull/13755 for how we are exposing the AzureContainerRegistryResource on each compute environment. We would have a similar property on IAzureComputeEnvironmentResource that returned the AzureUserAssignedIdentityResource associated with the compute environment.

cc @stepro