OpenAPI: Provide an attribute for ignoring parameters/properties

## Background and Motivation

When parameters or properties are set by middleware or otherwise managed internally, there should be a way to hide them from the generated OpenAPI spec.

The existing `ApiExplorerSettingsAttribute` can only be used on classes/methods, not individual parameters/properties.

## Proposed API

Provide an attribute similar to Swashbuckle's `[SwaggerIgnore]` that can be used on parameters and properties to exclude them from the generated OpenAPI spec.

```csharp
namespace Microsoft.AspNetCore.OpenApi;

[AttributeUsage(AttributeTargets.Parameter | AttributeTargets.Property)]
public sealed class OpenApiIgnoreAttribute : Attribute
{
}
```

## Usage Examples

```csharp
public sealed class CreateOrderRequest
{
public required string Name { get; init; }

[OpenApiIgnore]
public string? TenantId { get; set; }
}
```

```csharp
app.MapGet("/orders", ([OpenApiIgnore] TenantContext tenantContext) =>
{
// tenantContext is populated my middleware and should not appear in OpenAPI.
});
```

## Alternative Designs

Consumers can use OpenAPI transformers to remove these parameters/properties manually, but that requires each project to implement the same behavior.

Another option would be to extend an existing attribute, such as `ApiExplorerSettingsAttribute` or `ExcludeFromDescriptionAttribute`, to support parameters/properties.

## Risks

This could cause the generated OpenAPI spec to omit parameters or properties that clients actually need if the attribute is used incorrectly.