Test out avoiding strum and Cow

[alamb]

I wanted to see what avoiding strum and Cow would do to the API in the variant Read API, so I tried it out:

• [Variant] Initial API for reading Variant data and metadata apache/arrow-rs#7535

[mkarbo]

Nice, thank you!

I am OK with the Cow removal, we can assess whether we want to re-add it when we do the builder API later, we were worried about the inability to have an owned variant, but let's see if it becomes an issue - it's easy to re-add.

Will also comment this on my PR reg. Cow