AspNetCore.HealthChecks.Uris: Add option to specify payload

[hartmark]

It would be nice to be able to add custom payload like this:

    .AddUrlGroup(uriOptions: x =>
    {
        x.AddUri(new Uri(builder.Configuration.GetValue<string>("FooApiUrl")! + "/api/baz"));
        x.AddPayload(new { Foo = "bar" });
        x.UseGet();
    }, "MyApi")