`Assets::get_many`

[Wuketuke]

fixes #16244

Assets<A> should have get_many / get_many_mut functions just like Query<...> does. thats why I took the implementation of get_many_mut for Query as an inspiration.

i had to add an #[allow(unsafe_code)] above get_many_mut, so i want to know if thats permitted at that location

this pr also makes #16334 obsolete

Testing

I added two unit tests for get_many_mut (since get_many is trivial), but ill happily write more if there are some edge cases i overlooked (unsafe code is scarry)

[alice-i-cecile]

I think we should do this; Contentious is just for the use of unsafe (which I also think we should use here!).

[Wuketuke]

i dont understand why the ci is failing now i just merged 1 change from main into my pr, and havent touched the places where these errors get thrown now it passed just fine before the merge

[andriyDev]

This is broken at head currently https://discord.com/channels/691052431525675048/692572690833473578/1381857479520161803

I believe we're still trying to work out a fix.

[andriyDev]

Related issue: https://github.com/bevyengine/bevy/issues/19573