Proposal: Apply convert node rules to image edge cases

[jstone-lucasfilm]

Following up on #1776, this is a proposal to apply the rules of convert nodes to a common edge for the image node, where the referenced image and node have different data types.

@meshula notes that this would be better aligned with the rules for image edge cases in OpenUSD, so adopting this approach could improve the consistency of assets in the USD/MaterialX ecosystem.

If we move forward with this proposal, we should update the specification and codebase for MaterialX in tandem, so that the two remain aligned in each version of MaterialX.