Add AsValueEnumerable for Unity.Collections types

[nuskey8]

This PR implements UnityCollectionsExtensions, which is currently unimplemented. I added AsValueEnumerable() to Unity.Collections types, including all native containers and FixedList/FixedString.

[nuskey8]

Points to consider in implementation are as follows.

• Methods for Unsafe containers have not been implemented. Most users will not use these containers directly, so support for these types is not necessary.
• In principle, it is possible to obtain a ReadOnlySpan<T> from FixedList or FixedString, but the necessary information is hidden internally, so we will need to use Unsafe.As() to extract the contents. Implementing these is complex, so I have not implemented them as I have determined that the benefits are not worth it.

[neuecc]

Thank you!
I was planning to just use NativeArray/List, so I'm grateful that you've covered it so comprehensively!