FR: Expose asteroid spawn limitation values

[Durandal-Thoth]

Straightforward FR - expose view cone angle, range, and any other values used to cull asteroid spawns typically used to prevent asteroids from popping into existence. The FRED part of this is likely the more work-intensive half of this request if asteroid code isn't a spaghetti nightmare.

[Goober5000]

I took a brief look at the code and there is no view angle at all, only a check to see whether the asteroid would be on-screen or not. After talking a bit with Durandal, we narrowed down the request to two items which would satisfy the mission requirements that prompted this PR:

• A designated spawn area for asteroids, rather than letting them spawn anywhere in the asteroid field;
• A flag to turn off the view checks.

See also the video from #6230 which demonstrates how this feature is intended to be used.