Add support for populating secrets with keyvault resource

[davidfowl]

Today the key vault resource doesn't support adding secrets and it should. Bonus points if we can support referencing those secrets from projects.

From #2398 (comment)

Scenario 1: Pushing connection strings and secrets into keyvault.

var builder = DistributedApplication.CreateBuilder();

var db = builder.AddAzureCosmos("cosmos")
                .AddDatabase("db");

var sendgrid = builder.AddConnectionString("sendgrid");

var kv = builder.AddAzureKeyVault("kv")
                         .AddSecret(db) // Add the secret to keyvault called connectionstrings--cosmos
                         .AddSecret(sendgrid); // Add a connection string from config and push it into keyvault.

builder.AddProject<Projects.Api>("api").WithReference(kv); // Only need a keyvault reference

Scenario 2: Referencing Secrets from keyvault

var builder = DistributedApplication.CreateBuilder();

var kv = builder.AddAzureKeyVault("kv");
var cosmos = kv.GetConnectionString("cosmos"); // Get a secret called connectionstrings--cosmos
var secret = kv.GetSecret("secretstuff");

builder.AddProject<Projects.Api>("api")
           .WithReference(cosmos) // Push the connection string from keyvault into an environment variable
           .WithEnvironment("SomeEnv", secret); // Reference the secret from an env variable.

In scenario 2, the application doesn't have a reference to keyvault, the compute environment will pull from keyvault and push it into the application.

Scenario 3: Referencing Secrets from KeyVault already provisioned and using the KeyVault SDK in the service projects

var builder = DistributedApplication.CreateBuilder();

var kv = builder.AddAzureKeyVault("kv");

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