Make sdf::Model::CanonicalLinkAndRelativeName public

[azeey]

This member function is the only way to obtain the relative name of the canonical link when the link is implicitly nested, i.e, the //model/@canonical_link attribute is empty and the canonical link resolves to a link nested in a child model. While sdf::Model::CanonicalLink can be used to resolve the link, the returned sdf::Link* can only provide the local (non-relative) name of the link.

When constructing models, a downstream application may first recursively construct internal representations of all the links of a model before creating an internal representation of a model. When doing so, the application would need to identify one of the internal links as the canonical link and associate it with the internal representation of the model. In order to use sdf::Model::CanonicalLink for this task, it becomes necessary to track the mapping between sdf::Link* and the internal links of the application. Whereas if the relative name of the canonical link is available via sdf::Model::CanonicalLinkAndRelativeName, the application can query its internal list of links by the the relative name. It's reasonable to assume that the mapping from name to internal link is maintained by any application that consumes libsdformat so using sdf::Model::CanonicalLinkAndRelativeName avoids extra overhead.

[scpeters]

sounds reasonable to me; I initially made it private to avoid exposing :: syntax in sdf9, but I think it's fine to make it public in 11. aside from adding it to the migration guide; this look good to me

[azeey]

sounds reasonable to me; I initially made it private to avoid exposing :: syntax in sdf9, but I think it's fine to make it public in 11. aside from adding it to the migration guide; this look good to me

I have updated the migration guide in 031ffbb. I put it in the "Additions" section since it's being added to the public API. Let me know if I should put it in "Modifications" instead.