connectUser / connectApiUser: let server be a URL

[aronatkins]

When connectUser() and connectApiUser() are provided a server value that is a URL rather than a servers() name, consider treating that value as a Connect URL and call addServer() to add a server entry.

# existing use:
rsconnect::connectUser("alice", "connect")

# suggested enhancement:
rsconnect::connectUser("alice", "https://connect.company.com")

With this change, folks could add a new Connect account with a single call rather than the two that we require today.