We are currently special-casing yang-ext:mount in RESTCONF URL parsing, but have very little leverage in terms of encoding YangInstanceIdentifiers containing this addressing construct. This is not really a problem for XML codec, as it can easily produce the required namespace declarations, but JSON breaks down as we need to perform namespace->module name resolution in the EffectiveModelContext corresponding to the mount point.

We have some amount of support for this via RFC8528 schema crossing. Extend this support to process our legacy yang-ext:mount identifiers.