Add Sanity::Asset.create functionality

[dvmonroe]

We've got some bare bones of the Sanity::Asset class - but there's no functioning ability to create an asset

See https://www.sanity.io/docs/http-api-assets

[ajistrying]

Hey there 👋 , bouncing around GH looking around at projects that might need some extra hands. Is this still API being actively worked on? Would love to dig in more and help if so, thanks!

[dvmonroe]

Hey @ajistrying development is somewhat stagnant mainly because the Gem continues to work for most functionality offered by Sanity.

We deff welcome any contributions if you have the time!