Should be possible to provide sdf/urdf models as strings

[ggould-tri]

Currently the only way to add SDF/URDF models to a MultibodyPlant is via AddModelFromFile. However there are cases where a model is more easily provided in the form of a string or a stream. For example @avalenzu notes that a DRC team had the robot provide its model over the wire, and I am currently working with a robot (WLKATA Mirobot) that emits the link data used in its internal IK via its serial protocol.

It would probably make sense to provide AddModelFromStream and a trivial wrapper around that for AddModelFromString.

[jwnimmer-tri]

The libsdformat APIs don't really support streaming, so I just plan to add a string overload directly.

[SeanCurtis-TRI]

@jwnimmer-tri I have a branch where I started this as well. Are you actively tackling this? I note you elevated the priority. Depending on how far along you are, I can either finish mine or retire it.

[SeanCurtis-TRI]

Never mind; I just noticed the PR.

[EricCousineau-TRI]

FWIW Semi-relates #9500 (stuff on the wire, etc)

[jwnimmer-tri]

I added AddModelFromString but not the plural AddAllModelsFromString companion. I'll reopen the issue (at lower priority) to go back and add that, too.

[rpoyner-tri]

Is there a reason this issue is still open? the String APIs have been available for quite some time.