Abstract use of instance/document IDs and provenance for ingredient use

[brogdonm]

Currently, the SDK relies on assets to support XMP to properly be an ingredient. Since not all assets support XMP, would be beneficial to abstract this behavior, to allow for:

• Optional method for getting a new ID to use for instance/document ID

  This would allow an asset to provide a customized ID instead of one with xmp:*

• Setting an instance/document ID when signing an asset the first time
• Getting an instance/document ID from an asset
• Getting the provenance URI of a remote manifest

The ingredient.rs mod would be able to utilize these new methods to build out the ingredient information and would appropriately support remote manifests.

[gpeacock]

This is a great idea and on my backlog of things to do