Support pre-provisioned configuration for azure resources

[davidfowl]

We removed the ability to specify a connection string directly for arbitrary resources now that we have the AddConnectionString method. This adds a bit of friction when you want to directly specify the information for pre-provisioned to be used in development, while still allowing them to be provisioned during deployment.

[davidfowl]

Moving this to preview 5. The preview 4 mode of operation will be something like this:

var keyVault = builder.ExecutionContext.IsPublishMode ? 
        builder.AddKeyVault("kv") : 
        builder.AddConnectionString("kv");

builder.AddProject<Projects.Api>("api").WithReference(kv);

We'll add the ability to use an existing resource in preview 5

[mitchdenny]

For GA we'll do use AddConnectionString(...).

[captainsafia]

I'm taking a look at this in reaction to the feedback we received from the Aspire + Azure Functions work.

[captainsafia]

Closing this out for 9.1 as a bulk of the work has been done here.

Docs issue for this is over at dotnet/docs-aspire#2549.

We can consider additional enchancements like #7514 to this in the future.