Support specifying a ReferenceExpression to AddConnectionString

[davidfowl]

This enables the creation of a "Just in time" resource with a custom connection string with slots that have to be filled in from the outside.

var builder = DistributedApplication.CreateBuilder(args);

var endpoint = builder.AddParameter("endpoint");
var secret = builder.AddParameter("key", secret: true);

builder.AddOpenAI("oai", endpoint, secret);

builder.Build().Run();

static class ConnectionStringExtensions
{
    public static IResourceBuilder<IResourceWithConnectionString> AddOpenAI(
        this IDistributedApplicationBuilder builder,
        string name,
        IResourceBuilder<ParameterResource> endpoint,
        IResourceBuilder<ParameterResource> key)
    {
        var csb = new ReferenceExpressionBuilder();
        csb.Append($"Endpoint={endpoint.Resource};");
        csb.Append($"Key={key.Resource}");
        return builder.AddConnectionString(name, csb.Build());
    }
}

[danbopes]

Just ran into this today :(

Having to create the resource builder myself is a pain.