feat: `connect_user` S3 class

[karawoo]

Creates new S3 classes connect_user and connect_users. Functions that previously took a user GUID as an argument now can take either the GUID or the connect_user object.

This is still a draft while we talk through the right approach for phasing in this new object and moving away from the data frame-centric return object of get_users().

• For now get_users() still returns a data frame.
• get_users_list() returns a list of connect_user objects; get_users() just calls get_users_list() and converts to a data frame. I don't love this naming at all.
• Arguments that were previously named user_guid, owner_guid, etc. are now user/owner to reflect the fact that they can be either the GUID or object. This may be a pretty significant breaking change but it also doesn't seem right to keep _guid in there when it might not be a guid.

I'm a little tempted to deprecate passing the GUID strings with a warning, and then a few releases down the line remove support for passing the GUID. At the same time we can maybe swap get_users() to returning a list and remove/deprecate get_users_list(). Would love others' thoughts as they look at this though, I think there are a lot of potential options.

Checklist

• [ ] Does this change update NEWS.md (referencing the connected issue if necessary)?
• [ ] Does this change need documentation? Have you run devtools::document()?