bevy_dynamic_plugin: fix `unsafe_op_in_unsafe_fn` lint

[BD103]

Objective

• Part of #11590.

Solution

• Fix unsafe_op_in_unsafe_fn for bevy_dynamic_plugin.

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Changelog

• Added further restrictions to the safety requirements of bevy_dynamic_plugin::dynamically_load_plugin.

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I had a few issues, specifically with the safety comment on dynamically_load_plugin. There are three different unsafe functions called within the function body, and they all need their own justification / message.

Also, would it be unsound to call dynamically_load_plugin multiple times on the same file? I feel the documentation needs to be more clear.

[BD103]

It seems there is a deeper issue with bevy_dynamic_plugin after asking on Discord. While a more permanent fix should eventually be developed, I think this PR is trivial enough that it should still be merged.

I do honestly need help writing the text for that one unsafe block though. :)

[BD103]

I just rebased this branch and resolved the conflicts. :)