Add optional culture param to indexer of IStringLocalizer

## Summary

Now when `WithCulture()` is gone there is no simple method to get localized resource from different culture without changing the application global `CurrentUICulture`.

## Motivation and goals

In my ASP.NET Core App i I'm using translated URLs (stored in resource files). On every page I need to create "Alternate Links" tags that lead to the similar page with a translation. For this I'm iterating through supported cultures and getting translated link for every alternate lang page.

## In scope

Since `ResourceManagerStringLocalizer `internally uses ResourceManger's `GetString()` method that already supports specifying culture, we should add the option to specify such culture instead of relying on global CurrentUICulture.

## Out of scope

none

## Risks / unknowns

`ResourceManagerStringLocalizer.GetStringSafely()` already accepts specific culture so this amendment should be safe to use.

## Examples

I implemented this feature by creating my own inherited `ResourceManagerStringLocalizer` that adds such method.

Extra methods should be added to `IStringLocalizer` interface
```
LocalizedString this[string name, CultureInfo culture] { get; }
LocalizedString this[string name, CultureInfo culture, params object[] arguments] { get; }
```

and example implementation of the first one in `ResourceManagerStringLocalizer` (similar approach would apply to the second method)
```
public virtual LocalizedString this[string name, CultureInfo culture]
{
get
{
if (name == null)
{
throw new ArgumentNullException(nameof(name));
}

var value = GetStringSafely(name, culture);

return new LocalizedString(name, value ?? name, resourceNotFound: value == null, searchedLocation: _resourceBaseName);
}
}
```

I can submit PR if you are happy with the suggestions.