Proposal: Apply convert node rules to image edge cases

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.

Start by reviewing issue #1776 and the existing specification and implementation of `convert` and `image` nodes, then compare the proposed edge-case behavior with OpenUSD. Done means the specification and MaterialX codebase apply aligned rules for image edges with differing data types in the relevant version.

Written by the indexing model from the issue text.