impl VisitAssetDependencies for bevy_platform::collections::HashMap

[Strikeless]

Objective

This PR adopts what's left of #14162, implementing VisitAssetDependencies for HashMap. This was implemented for HashSet's in #20735.
This accomplishes the ability to mark Handle<T>/UntypedHandle values of a hashmap as dependencies of a struct deriving Asset using #[dependency] on the hashmap field.

Solution

impl VisitAssetDependencies for HashMap

Testing

• Verified to build using cargo build at bevy root
• verified to not crash in use on a cherry-pick to 0.17.3 release

I believe there is no need for more testing, given the simplicity of the change and the similarity of the two added impl blocks to previous ones.

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[andriyDev]

Hmmm I'm a little nervous to do this, because it means that keys of a hashmap won't be included in VisitAssetDependencies. I feel like forcing all uses of HashMap to manually impl VisitAssetDependencies is better than accidentally doing the wrong thing if someone has HashMap<Handle<A>, Handle<B>>.

[Strikeless]

Fair concern I hadn't thought about. I feel like manually implementing VisitAssetDependencies for concrete hashmap types is a bit clunky still, although for sure better than messing up a HashMap<Handle<A>, Handle<B>>.
Due to orphan rules you'd need to wrap hashmap in a newtype if you wanted to impl VisitAssetDependencies with a key/value type external to Bevy. I wonder if there's a practical solution to this, or if I should close the PR and stick with newtyping and manual impl?