Reduce Allocations in ValidateSignature

[eerhardt]

When we need to create an enumerable with a single element in it, we are allocating a new List<SecurityKey> { key }. This allocates both the list, and then allocates a 4 element array backing the list (because this syntax is C# sugar for just calling Add on the collection).

Instead, use a collection expression which optimizes the single element case.

There was one method in Saml that had this pattern in it, but was unused. I opted to remove it. Please let me know if it should be preserved for some reason.

Using an internal benchmark with 1,000 iterations, these allocations will be cut in half:

[eerhardt]

See dotnet/roslyn#71147 for what Roslyn does in these cases. It generates a <>z__ReadOnlySingleElementList class that is used for the single element case.