Enhance external parameters article with 9.2 AddConnectionString capabilities

[davidfowl]

We just merged dotnet/aspire#7826 into 9.2 which adds an overload of AddConnectionString that supports passing in a reference expression.

There are a few key things this enables:

1. Creating adhoc connection strings made from various parameters:

   var openAiKey = builder.AddParameter("openaikey", secret: true);
   var oaiCs = builder.AddConnectionString("llm", ReferenceExpression.Create($"Endpoint=https://api.openai.com/v1;Key={openAiKey}"));

2. Appending to existing connection strings.

    var builder = DistributedApplication.CreateBuilder(args);
   
    var postgres = builder
        .AddPostgres("postgres")
   
    var database = postgres.AddDatabase("db");
   
    var cs = builder.AddConnectionString("database", ReferenceExpression.Create($"{database};Include Error Details=true"));
   
    var backend = builder
        .AddProject<Projects.Backend>("backend")
        .WithReference(cs)
        .WaitFor(cs);
   
    builder.Build().Run();

   Waiting for a connection string will wait for the database implicitly.

This article should be updated to show the new scenarios.