feat: read session token from file, if present #518
Description
An upcoming release will include an env var CONNECT_CONTENT_SESSION_TOKEN_FILE, which points to a file that can be read in and contains the equivalent of CONNECT_CONTENT_SESSION_TOKEN, except that it gets refreshed periodically so that a long-running process can always have a valid session token. Wherever connectapi currently reads CONNECT_CONTENT_SESSION_TOKEN, it should instead look for the existence of CONNECT_CONTENT_SESSION_TOKEN_FILE, and if set, read in the file it references, falling back to the other env var to support older versions of Connect.