Fix memory leak in dynamic ECS example

[james-j-obrien]

Objective

• Fix a memory leak in the dynamic ECS example mentioned in The dynamic example is scary and needs a revamp #11459

Solution

• Rather than allocate the memory manually instead store a collection of Vec that will be dropped after it is used.

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I must have misinterpreted OwningPtrs semantics when initially writing this example. I believe we should be able to provide better APIs here for inserting dynamic components that don't require the user to wrangle so much unsafety. We have no other examples using insert_by_ids and our only tests show it being used for 1 or 2 values with nested calls to OwningPtr::make despite the function taking an iterator. Rust's type system is quite restrictive here but we could at least let OwningPtr::new take non u8 NonNull.

I also agree with #11459 that we should generally be trying to simplify and clarify this example.

[nicopap]

LGTM