feat: return a list of Content class objects from search_content()

[toph-allen]

Intent

Instead of returning a list of lists from search_content(), return a list of Content class objects. This allows users to use existing content-related functions with the list via purrr.

Fixes #452
Fixes #448

Approach

This is pretty minimal so far — just changing the class of the returned object.

Using purrr to act upon the list of content is not an ideal workflow, but this is the first step towards something more user-friendly. See the original issue.

Documentation is not ideal. The Connect class does not document available fields, and the fields documented on get_content() are out of date. I filed #461 to track this.

Checklist

• Does this change update NEWS.md (referencing the connected issue if necessary)? [Not needed]
• Does this change need documentation? Have you run devtools::document()?

[marcosnav]

Looks good!

[jonkeane]

This is a bit of an odd example, no? I suspect it's there to show that (currently) one should use purrr::keep(...) for filtering, but in this case, you would actually want to do my_content <- search_content(client, q = "owner:@me type:rmd-shiny") to do this.

There's an arc of work to make the list more-easily-filterable that we should take up (later/next), but IMO we should document the stuff we're building now with how it should be used today, even if there might be other ways to do / other tasks folks might do with stuff like this in the future.

[toph-allen]

Good point!! Perhaps I should use the longer example I used in GitHub PRs that uses a comparison that isn't possible in the filter API at the moment.

[edit] And/or just remove this example.