Document how to customize Azure Container Apps deployment in C#

[davidfowl]

Aspire 9 makes it possible to customize the container apps definition when deploying to ACA using C# (the new Azure CDK)

dotnet/aspire#5470

There are 2 new methods:

1. PubishAsAzureContainerApp for container resources
2. PublishAsAzureContainerApp for project resources

This exposes a callback that allows users to set properties on the container app definition. It also flips azd into a mode where the bicep for container apps is generated by the apphost.

Relies on a future version of azd