Enhance OpenAPI support in Minimal API with ProducesDefault for default responses

## Summary

Allow developers to set the `default` response for OpenAPI operations with the `Produces()` family of extensions methods from `Microsoft.AspNetCore.OpenApi` to achieve parity with MVC's `[ProducesResponseType]`.

## Motivation and goals

Being able to set the default response for an operation is useful as it demonstrates the default response for unexpected/undocumented operation responses. A common response type for `default` is `ProblemDetails`, as this usually is the standard type for API errors.

Proper use of `default` is also very important for API client generation, because API Client generators can then ensure that any other errors would always be typed to a specific type like `ProblemDetails`, which is a common pattern for error responses in REST APIs. I generate API clients quite often, and often run into issues where undocumented/unexpected error codes aren't deserialized into a correct type because of a missing `default` response type.

Setting `default` is easy to do with the MVC approach:

Click here to see the MVC approach

```csharp
[HttpGet("{id:int:min(1)}")]
[ProducesResponseType(StatusCodes.Status200OK)]
[ProducesResponseType(StatusCodes.Status403Forbidden)]
[ProducesResponseType(StatusCodes.Status404NotFound)]
[ProducesDefaultResponseType)]
public ActionResult GetTalk(int id)
{
// Code here
}
```

which could result in the following OpenAPI document (Parts like problem details and other irrelevant code has been removed for brevity):

```yaml
paths:
/talks/{id}:
get:
operationId: "GetTalk"
parameters:
- name: "id"
in: "path"
required: true
schema:
type: "integer"
responses:
'200':
content:
application/json:
schema:
$ref: "#/components/schemas/Talk"
'403': # stuff
'404': # stuff
'default': # stuff
```

You can already use `[ProducesDefaultResponseType]` with Minimal API's, but it's not as discoverable, nor as much as a first-class citizen, as the `Produces` family of methods from the `Microsoft.AspNetCore.OpenApi` package:

**Attributes with Minimal API:**
```csharp
app.MapGet("/todos/{id}",
[ProducesResponseType(StatusCodes.Status200OK)]
[ProducesResponseType(StatusCodes.Status403Forbidden)]
[ProducesResponseType(StatusCodes.Status404NotFound)]
[ProducesDefaultResponseType]
async (TodoDb db, int id) => /* Code here */)
```

**Extension methods with Minimal API and default:**
```csharp
app.MapGet("/todos/{id}", async (TodoDb db, int id) => /* Code here */)
.Produces(StatusCodes.Status200OK)
.ProducesProblem(StatusCodes.Status403Forbidden)
.ProducesProblem(StatusCodes.Status404NotFound)
```

There is no way to set the default response for an operation using extension methods. For this, you'd need an Operation Filter or an attribute, which doesn't mix very well with the extension methods. Also, `ProducesDefaultResponseType` is of the MVC namespace, which might be considered odd when using the Minimal API approach.

**Extension methods with Minimal API and attribute for default:**

```csharp
app.MapGet("/todos/{id}",
[ProducesDefaultResponseType]
async (TodoDb db, int id) => /* Code here */)
.Produces(StatusCodes.Status200OK)
.ProducesProblem(StatusCodes.Status403Forbidden)
.ProducesProblem(StatusCodes.Status404NotFound)
```

Therefore, I'd like to add something like `ProducesDefault` to the `Microsoft.AspNetCore.OpenApi` package to allow developers to set the default response for an operation using extension methods.
## In scope

1. Add a `ProducesDefault` extension method to the `Microsoft.AspNetCore.OpenApi` package.
1. Perhaps this defaults to `ProblemDetails`?
2. It should also allow for a content type and schema to be set.
2. This would work on specific endpoints.
3. This could work on endpoint groups, too.
4. Perhaps this should be enabled by default for all applications, as you usually want this enabled for every endpoint, decreasing the amount of boilerplate code needed to get this to work.
1. Consider allowing a developer to configure the `default` settings in `AddOpenAPI`, like setting the content type and/or schema (like problem details or something else) globally.

## Out of scope

None come to mind right now.

## Risks / unknowns

One risk is considering my other issue of adding `Description` to the `Produces` family of methods: #57963, as this would be more ergonomic than current options.

I've created a design proposal for this here: #58724 , but there is an issue of source incompatibility as a string description would clash with the `string contentTypes, params string[] additionalContentTypes` parameters of the `Produces()` family of methods.

If that design proposal is accepted, we should consider how to make `ProducesDefault` work with `Description` as well.

This is important, as I recently implemented setting `Description` on `ProducesResponseType` and that family of attributes for MVC (#55656), so we should try to decrease the amount of friction for adding better Description support to Minimal API's, too.

## Examples

### Setting the default response for an operation

```csharp
app.MapGet("/todos/{id}",
async (TodoDb db, int id) => /* Code here */)
.Produces(StatusCodes.Status200OK)
.ProducesProblem(StatusCodes.Status403Forbidden)
.ProducesProblem(StatusCodes.Status404NotFound)
.ProducesDefault();
//.ProducesDefault();
//.ProducesDefault("application/xml");
```

### Setting the default response for an endpoint group

```csharp
app.MapGroup("/todos").MapTodosApi().ProducesDefault();
```

### Customizing global default response

```csharp
builder.Services.AddOpenApi(x => x.DefaultResponseType = typeof(CustomApiErrorClass));
```