feat: new function `set_content_image()`, deprecate old `set_image_*` functions

[toph-allen]

Fixes #294

• Deprecate set_image_path(content, path), set_image_url(content, url), set_image_webshot(content).
• Add a new function, set_content_image(content, path). The new function can handle a local file path or a remote URL. If the path points to a local file that exists, it will use that. Otherwise, if it is an https or http URL, it will attempt to download that image to a temporary file and use that (the old behavior of set_image_url().

In a future PR to the connect repo, set_image_webshot() will become a recipe in the SDK Cookbook.

Note: this wasn't based off the 0.3.0 release, so the NEWS will have a merge conflict once that PR has merged. Not sure why I started it from main, but I did 🤷🏻.

[toph-allen]

Oops, accidentally closed with a force push.