[Aspire 10] - Obsolete AddProject<>, replace with AddDotnetApp

## Background and Motivation

We should consider aligning the APIs for dotnet with JS/Python - and removing the need for project references. We would have to handle any code gen or build ordering that dotnet itself handles via the projectrefs now.

It would be great to have dotnet line up w the other langs -- we can still enable the powerful dotnet experience we have today but with a unified API. This also solves one of my personal gripes of having to mess around with project references by hand in vs code lol.

Additionally, this would work with single file apps!!!! YAY

## Proposed API

Please provide the specific public API signature diff that you are proposing. For example:
```diff
namespace Aspire.Hosting;

- public static ApplicationModel.IResourceBuilder AddProject(
- this IDistributedApplicationBuilder builder
- string name
- string projectPath);
+ public static ApplicationModel.IResourceBuilder AddDotnetApp(
+ this IDistributedApplicationBuilder builder,
+ string name,
+ string projectFilePath); (could overload for string csharpFilePath or something)

```

## Risks

Definitely gonna require work across the stack. Obsoleting an API that _everyone_ uses is pretty messy, too, but we should do this before it's too late lol

Start by reviewing the existing AddProject API and the proposed AddDotnetApp signature on IDistributedApplicationBuilder. Then trace how ProjectResource and the proposed DotnetAppResource participate in project references, code generation, and build ordering. Done requires an agreed API and migration approach that addresses the cross-stack and obsoletion risks.

Written by the indexing model from the issue text.