Spawn vehicle at requested orientation

[chapulina]

Part of

• Document / update coordinates and units #48

Requires

• Optional orientation when spawning entity using spherical coordinates gazebosim/gz-sim#1263
• Document pose + spherical coordinates combination on spawn gazebosim/gz-msgs#212
• Forward-port: 6 ➡️ 7 (main) gazebosim/gz-sim#1293

This is a subset of

• Fix coordinate frames on state message #81

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Make use of the roll, pitch and heading fields on the init message when spawning a vehicle.

I also changed the test to run paused, so we don't have the physics affecting the spawn pose and we can check it more precisely.

This takes an alternative approach to #81. Instead of rotating the model to face North by default, we introduce offsets when receiving spawn commands. This means we must be mindful of the fact that the vehicle faces West by default all the time when looking at poses, which is extremely error prone. I'd much rather change the model to match NED / FSK by default, but as explained in #48 (comment), there are physics issues with using a Z-down model that we'll need more time to track down.

[chapulina]

I forgot that the orientation is in NED, so this can't escape #81 . See updated description above.

[chapulina]

Upstream's forward-port was merged. After tonight's nightlies are built, we should retrigger CI here.

[chapulina]

Ahh last night's ign-gazebo7 nightly failed:

https://build.osrfoundation.org/job/ign-gazebo7-debbuilder/159/

[chapulina]

Yay, the latest nightlies are out and CI is 🟢

[arjo129]

Thanks for these docs.