Add OptionalSystemParam and OptionalSystemParamFetch trait

[JonahPlusPlus]

Objective

• There is no way to implement SystemParam for Option<T>, due to orphan rules.

Solution

• Instead of trying to implement SystemParam for Option<T>, users can implement OptionalSystemParam for T and OptionalSystemParamFetch for TState.
• SystemParam is implemented for all Option<T: OptionalSystemParam.

---

Changelog

This section is optional. If this was a trivial fix, or has no externally-visible impact, you can delete this section.

• Added OptionalSystemParam and OptionalSystemParamFetch.
• Changed internal Option<T> system params to use OptionalSystemParam and OptionalSystemParamFetch.

[JonahPlusPlus]

@james7132 I have a solution for your concern. You can check out the PR on my fork here.

Basically, the idea is to implement OptionalSystemParam for your system param. Then, instead of implementing SystemParamFetch for your state, implement OptionalSystemParamFetch.

Implementing OptionalSystemParam and OptionalSystemParamFetch for T and TState will automatically implement SystemParam for T and Option<T>. This works by automatically implementing SystemParamFetch for TState and Option<TState>, which means that you can create optional system params that use resources by using Option<ResState<T>>.

If this is acceptable to you, I'll merge it to this PR.

[JonahPlusPlus]

@alice-i-cecile I'm closing this PR because I want to redo it once #6865 is merged (if you could review and merge it that would be appreciated). With the simplified trait structure, it will be easier to add OptionalSystemParam and ResultableSystemParam at once.

ResultableSystemParam is an idea @JoJoJet and I discussed briefly on discord. Basically, by implementing it you can use Result<T, E> as a system param. It also creates a sort of fallback structure:

• Implementing ResultableSystemParam implements SystemParam for Result<T, E>, Option<T> and T
• Implementing OptionalSystemParam implements SystemParam for Option<T> and T
• Implementing SystemParam just provides T

This gives users precise control over error handling.