Add extension methods to IEnumerable<OptionsDescriptor<T>>

[yishaigalatzer]

It is currently rather cumbersome to configure options in any of the lists in MvcOptions. The user has to write selector code to get to the descriptor objects, and then get the actual instance. For example

services.ConfigureOptions<MvcOptions>(options => 
    options.InputFormatters.Single(fd => fd.Instance is JsonInputFormatter).Instance.XXXX

Instead we should have a set of extension methods allowing easy access to the descriptors/instances

Here is an initial suggestion - @rynowak lets discuss and update

public static TInstance InstanceOf<TInstance>(this IEnumerable<OptionsDescriptor<TInstance>> collection);
public static TInstance InstanceOfOrDefault<TInstance>(this IEnumerable<OptionsDescriptor<TInstance>> collection);
public static IEnumerable<TInstance> InstancesOf<TInstance>(this IEnumerable<OptionsDescriptor<TInstance>> collection);
public static void RemoveTypesOf<TDescriptor>(this IList<OptionsDescriptor<TDescriptor>> collection);

perhaps we should also consider a replace in place extension, so a customized options can come in the exact same spot the one it replaced is in.
#1744

[tonysneed]

I'm ready to do this one now, and I can submit a PR for it.

[tonysneed]

It looks like extensions to IEnumerable<OptionDescriptor> aren't going to provide the desired result. For example this method:

public static TInstance InstanceOf<TInstance>([NotNull] this 
    IEnumerable<OptionDescriptor<TInstance>> descriptors)

Called by this code:

var formatter = new MvcOptions().InputFormatters.InstanceOf<JsonInputFormatter>();

Will produce a CS1929 compilation error: The extension method requires a receiver of type IEnumerable<OptionDescriptor<JsonInputFormatter>>.

I would like to suggest the following (for both input and output descriptors):

public static TInstance InstanceOf<TInstance>([NotNull] this
    IEnumerable<InputFormatterDescriptor> descriptors)

[yishaigalatzer]

You can alternatively define the IOptionsDescriptor<T> interface and use InstanceOf<T>(IEnumerable<object>)

e.g.

public class InputFormatterDescriptor : OptionDescriptor<IInputFormatter>, 
                   IOptionDescriptor<IInputFormatter>
{
     ...
}

and

public interface IOptionDescriptor<out TOption>
{
    Type OptionType { get; }
    TOption Instance { get; }
}

and

public class OptionDescriptor<TOption> : IOptionDescriptor<TOption>
{
    ....
}

and finally

public static TInstance InstanceOf<TInstance>([NotNull] this IEnumerable<IOptionDescriptor<object>> descriptors) 
{
    return default(TInstance);
}