Akamai Technologies Inc. has agreed to team up with AT&T Inc. to begin delivering Web content for the telecommunications giant. The deal ends a longtime rivalry between the Cambridge company and AT&T in the lucrative business of content delivery network services, which allow video and other information to be sent over the Internet. It also gives Akamai access to a vast new customer base. AT&T will start using Akamai technology in its North American market and then expand globally over the next year.