More constructors and member functions for `vk::raii::PluralType`

From #2413:

It would be nice to implement an `operator*()` or `::value()` member function for `vk::raii::PluralType` that returns a `std::vector` (note: **no `::raii`**); this would recursively call `operator*()`.

And similarly to construct the plural types from an `ArrayProxy&&` (note temporary so that the resultant container takes ownership of the arguments), and also note **again no `::raii`**). This would facilitate interoperation with other libraries.

Start by tracing the vk::raii::PluralType definitions and the related work described in #2413. Check how plural types currently construct and expose their values, then verify that recursive conversion to std::vector and construction from rvalue ArrayProxy support the stated interoperability requirements.

Written by the indexing model from the issue text.