Adding the raw text input formatter

## Background and Motivation

Sometimes we need to read the raw request body as `string` with content-type `text/plain`, it would help us handle these requests more easily

## Proposed API

```csharp
namespace Microsoft.AspNetCore.Mvc.Formatters;

public sealed class RawTextInputFormatter : TextInputFormatter
{
public RawTextInputFormatter()
{
SupportedMediaTypes.Add(MediaTypeHeaderValue.Parse("text/plain"));

SupportedEncodings.Add(Encoding.UTF8);
SupportedEncodings.Add(Encoding.Unicode);
}

public override async Task ReadRequestBodyAsync(InputFormatterContext context, Encoding encoding)
{
using var reader = context.ReaderFactory(context.HttpContext.Request.Body, encoding);
var rawContent = await reader.ReadToEndAsync();
return await InputFormatterResult.SuccessAsync(rawContent);
}
}

```

## Usage Examples

``` csharp
services.AddControllers(options =>
{
options.InputFormatters.Add(new RawTextInputFormatter());
});
```

## Alternative Designs

Maybe also we could use `StringInputFormatter` for the type name since there's a `StringOutputFormatter`, and `PlainTextInputFormatter` can be also treated as a candidate

## Risks

I think it should be fine since it's a new type, since .NET 8 is a major release, maybe we could also include it as one of the default formatters