Move asset_input.rb into this gem

[gburgett]

The wcc-styles gem provided a simple_form extension that made it easy to link tessa-client :asset fields to Dropzone. We should copy that extension into the tessa-client gem, and have it auto-register in an Engine initializer if simple form is present.

This would make the tessa-client v2.x gem into a true one-stop-shop for convenience wrappers around ActiveStorage uploads. We still need it anyways for the Dropzone direct-upload initialization via POST /tessa/uploads, as well as the Dropzone configuration inside of tessa/index.js.coffee. So I don't think we're getting rid of this gem anytime soon.