Add `EntityMut::get_mut_by_id_unchecked`

[alice-i-cecile]

I still think there should be a get_mut_by_id_unchecked. The apis added in #15593 could end up allocating in some tight loops which would be a perf footgun. The unchecked version would allow users to implement their own versions in userspace that could reuse the allocations.

Originally posted by @hymm in #15577

[BenjaminBrienen]

I intend to work on this if no one else gets to it first

[mockersf]

it was also suggested that a variant checking unicity though a hashset could be better: https://github.com/bevyengine/bevy/issues/15577#issuecomment-2387108509