Introduce the Results<TResult1, TResult2, TResultN> union types

[DamianEdwards]

This PR introduces just the Results<TResult1, TResult2, TResultN> union types themselves. The types are code generated via a tool, along with their unit tests.

When #40646 lands we can update the in-box IResult implementations to describe themselves via emitted metdata, and update the Results<TResult1, TResult2, TResultN> union types to pass through that metadata from the TResultN type args, along with introducing the new Results.Typed factory methods for creating results and preserving the concrete types.

Until then, the union types allow all the possible IResult return types of a route handler delegate to be declared explicitly and thus checked at compile for type safety, e.g.:

app.MapGet("/todos/{id}", async Task<Results<OkObjectHttpResult, NotFoundObjectHttpResult>> (int id, TodoDb db) =>
{
    return await db.Todos.FindAsync(id)
        is Todo todo
            ? (OkObjectHttpResult)Results.Ok(todo)
            : (NotFoundObjectHttpResult)Results.NotFound();
});

Fixes #40672

[brunolins16]

Is not possible (or make sense) use a T4 template instead of introduce a new project to generate the code?

[DamianEdwards]

Is not possible (or make sense) use a T4 template instead of introduce a new project to generate the code?

I think to date the repo has used console apps for code generation in numerous places (Kestrel, Developer Exception Page, etc.) so I followed that pattern. I'm not super fussed other than it would mean more work to convert what I've done in this PR to use T4 instead, but there'll still be a mix in the repo.

[halter73]

Can we also add a test that fails if the generator and generated code get out of sync like Kestrel's GeneratedCodeIsUpToDate test?

[DamianEdwards]

Done!

[halter73]

Suggested change

	writer.WriteIndentedLine("public async Task ExecuteAsync(HttpContext httpContext)");
	writer.WriteIndentedLine("{");
	writer.WriteIndentedLine(2, "ArgumentNullException.ThrowIfNull(httpContext, nameof(httpContext));");
	writer.WriteLine();
	writer.WriteIndentedLine(2, "if (Result is null)");
	writer.WriteIndentedLine(2, "{");
	writer.WriteIndentedLine(3, "throw new InvalidOperationException(\"The IResult assigned to the Result property must not be null.\");");
	writer.WriteIndentedLine(2, "}");
	writer.WriteLine();
	writer.WriteIndentedLine(2, "await Result.ExecuteAsync(httpContext);");
	writer.WriteIndentedLine("public Task ExecuteAsync(HttpContext httpContext)");
	writer.WriteIndentedLine("{");
	writer.WriteIndentedLine(2, "ArgumentNullException.ThrowIfNull(httpContext, nameof(httpContext));");
	writer.WriteLine();
	writer.WriteIndentedLine(2, "if (Result is null)");
	writer.WriteIndentedLine(2, "{");
	writer.WriteIndentedLine(3, "throw new InvalidOperationException(\"The IResult assigned to the Result property must not be null.\");");
	writer.WriteIndentedLine(2, "}");
	writer.WriteLine();
	writer.WriteIndentedLine(2, "return Result.ExecuteAsync(httpContext);");

It's a little bit more efficient if we skip the tail await at the small cost of losing the stack frame if there are any exceptions.