sdformat: Should allow merge include URDF files to be included via include tags

[EricCousineau-TRI]

Steps:

• Ensure we have negative test showing that merge-include URDF inclusion via include tag fails fast (rather than allowing SDFormat to convert)
• Ensure we have necessary changes to libsdformat interface API to support including URDF files (without loss of information)

[EricCousineau-TRI]

This will be resolved by incorporating gazebosim/sdformat#768

[jwnimmer-tri]

I see that the ignitionrobotics pull request has been merged. What is the next step to resolve this issue?

Model composition is an important feature in Drake. I'd like to advocate that we prioritize OpenRobotics to finish the model-includes feature that has been an ongoing project for several years, before we start working on new projects.

[EricCousineau-TRI]

There is still pending work for this as tracked on SDFormat project board:
https://github.com/orgs/ignitionrobotics/projects/6

Outstanding work:

• Ensure that our usage of Interface API accommodates all of Drake's parseable model types. See [parsing] Add support for using merge-includes with custom parsed models #16727
• Ensure we have conversion scripts for model directives to SDFormat composition. Will require redux of [parsing] Script for converting model directives into sdformat files aaronchongth/drake#1