WithMetadata support

[mitchdenny]

Code sample:

var builder = DistributedApplication.CreateBuilder(args);
var pg = builder.AddPostgres("mypostgres")
                .WithMetadata("azd-resource-flavour", "flexibleserver")
                .WithMetadata("azd-postgres-flexible-server-sky", "P1")
                .WithMetadata("azd-resource-location", "westus3");

This is alread implemented in this PR: #1619

Purpose is to support underlying communication with deployment tools.