feat: read session token from file, if present

[nealrichardson]

Intent

Closes #518

Approach

Write a failing test, make it pass, update the docs

Checklist

• Does this change update NEWS.md (referencing the connected issue if necessary)?
• Does this change need documentation? Have you run devtools::document()?
• Does this change resolve integration test failures for a preview build of Connect? If so (and if you are on the Connect team), please be sure to update the Connect changelog to reflect that users will need to upgrade their version of connectapi.

[tdstein]

I don't know R primitives very well. Does the nzchar equality check handle conditions where the file specified at CONNECT_CONTENT_SESSION_TOKEN_FILE is missing? Also, will if (is.null(content_session_token)) evaluate to false if the file is empty?

[nealrichardson]

nzchar is handling "is the env var set and not empty". If the file it points to does not exist, readLines() will throw an error. If the file exists but is empty, that would return a 0-length string, which would be an invalid token but not NULL so it wouldn't fall back to checking the CONNECT_CONTENT_SESSION_TOKEN env var.

We could be more defensive around those, I suppose, but what other than raising a different error message would we do? If the session token file is invalid, why should we expect anything else to work?

[tdstein]

yeah yeah, I just wanted to make sure that it handled empty files in the same way as the Python implementation for consistency.