Allow File Rules to leverage file format metadata

Currently the OCIO File Rules are only based on the path string for the media. This was done intentionally since dealing with file format metadata was deemed too complex for the first pass at the feature. However, it's definitely the case that there often is metadata in the file header that could be useful for determining what the appropriate color space is and it would be useful for the File Rules to be able to leverage it.

We do not want to add any dependencies for file format libraries on to OCIO to support this feature, so it needs to be implemented in such a way that the host application parses the files and provides the metadata in some neutral representation, such as a pair of field & value strings. OpenImageIO has some techniques for representing file-format agnostic metadata that might be a good model for how this could work (without adding a dependency on OIIO).

Of course the metadata in file headers is often wrong, so there needs to be sufficient control in the File Rules to control the relative precedence of what is in the file relative to the existing path-based rules.

Here is a presentation that [discussed OpenEXR](https://docs.google.com/presentation/d/1sY55UHYi2AJF6EPYDYY-mvaJUCzZCkWisrj-a9yhg2w/edit?usp=sharing) that is related.

Iniziare esaminando le OCIO File Rules esistenti e la presentazione OpenEXR collegata. Definire come vengono rappresentati i metadati di campo e valore forniti dall’host e come viene controllata la loro precedenza rispetto alle regole basate sul percorso. Il lavoro è completo quando File Rules può utilizzare metadati neutrali senza aggiungere dipendenze da librerie per i formati di file.

Scritto dal modello di indicizzazione a partire dal testo della issue.