A new authentication approach has been added to the coordinator, so to better
represent the options, this commit changes the configuration to accept
different records, each for different authentication approaches.

* gnu/services/guix.scm (guix-build-coordinator-agent-configuration-uuid,
guix-build-coordinator-agent-configuration-password,
guix-build-coordinator-agent-configuration-password-file): Removed
procedures.
(guix-build-coordinator-agent-password-auth,
guix-build-coordinator-agent-password-auth?,
guix-build-coordinator-agent-password-auth-uuid,
guix-build-coordinator-agent-password-auth-password,
guix-build-coordinator-agent-password-file-auth,
guix-build-coordinator-agent-password-file-auth?,
guix-build-coordinator-agent-password-file-auth-uuid,
guix-build-coordinator-agent-password-file-auth-password-file): New
procedures.
(guix-build-coordinator-agent-shepherd-services): Adjust to handle the
authentication field and it's possible record values.
* doc/guix.texi (Guix Build Coordinator): Update documentation.