C#: Refactor Secret Dispose usage

Refactor `Secret` and `ProtectedMemorySecret` `Dispose` usage to avoid requiring implementing classes from having to suppress the finalizer. Refer to https://docs.microsoft.com/en-us/dotnet/standard/garbage-collection/implementing-dispose.

Abstract class should end up looking like:
```c#
protected abstract void Dispose(bool disposing);
public void Dispose()
{
this.Dispose(true);
GC.SuppressFinalize(this);
}
```

Will need to test if the new allocator instance is needed as in the current implementation.