Support for SDF <sim_time> element

[jrutgeer]

Support to set the initial simulation time through a command line argument was added in Garden: #1801.

However, there also is the SDF element <sim_time> which has the same purpose. Based on this RSE question I conclude that the SDF element does not work (though unconfirmed: I can't test this right now. Also the RSE question refers to Fortress, but I did not find pull requests other than #1801 so I presume it is still relevant).

Based on this answer to a similar RSE question, I conclude that the SDF element does work for Gazebo Classic.

[azeey]

The whole <state> tag is not implemented yet. This is related to #2318 and gazebosim/sdformat#1461