wrap LanguageTags type in new AnyOrSome type to support wildcards

[robjtede]

PR Type

Fix

PR Checklist

• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• A changelog entry has been made for the appropriate packages.
• Format code with the latest stable rustfmt.
• (Team) Label with affected crates and semver status.

Overview

Introduce new AnyOrSome (bikeshedding welcome) wrapper type to support wildcard items in header types where the backing type has no concept of wildcard/any types.

Used initially in AcceptLanguage but for consistency it might make sense to re-work Accept (which does know about wildcards).

fixes #2434

[robjtede]

cc @Dexterp37

[finnbear]

Introduce new AnyOrSome (bikeshedding welcome)

I assume you are trying to avoid a compound-word name. Here are some ideas:

1. Preference<T>

enum Preference<T> {
    Any,
    Specific(T)
}

2. Just use Option<T> where None is taken to mean nothing specific i.e. any
3. Other considered enum names: Specificity<T>

[robjtede]

1. I kinda like Preference<T>
2. Nah, the None variant would be confusing.
3. it's alright, just a bit harder to spell/type than preference