Add IReadOnlyCollection<T> as a optimised path for LINQ methods (e.g. .Count()) #135266

using System.Collections;

var rocollection = new ReadOnlyCollection<int>([1, 2, 3, 4, 5]);
var count = rocollection.Count();
Console.WriteLine(count);
Console.WriteLine($"IsCountInvoked:{rocollection.IsCountInvoked}; IsGetEnumeratorInvoked:{rocollection.IsGetEnumeratorInvoked}");

class ReadOnlyCollection<T> : IReadOnlyCollection<T>
{
    private readonly List<T> _list;

    public ReadOnlyCollection(IEnumerable<T> enumerable)
    {
        this._list = new(enumerable);
    }

    public bool IsCountInvoked { get; private set; } = false;
    public bool IsGetEnumeratorInvoked { get; private set; } = false;

    public int Count
    {
        get
        {
            this.IsCountInvoked = true;
            return this._list.Count;
        }
    }

    public IEnumerator<T> GetEnumerator()
    {
        this.IsGetEnumeratorInvoked = true;
        return this._list.GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator() => this.GetEnumerator();
}