services: guix: Add guix-build-coordinator-queue-builds-service-type.

* gnu/services/guix.scm (<guix-build-coordinator-queue-builds-configuration>):
New record type.
(guix-build-coordinator-queue-builds-configuration,
guix-build-coordinator-queue-builds-configuration?,
guix-build-coordinator-queue-builds-configuration-package,
guix-build-coordinator-queue-builds-configuration-user,
guix-build-coordinator-queue-builds-coordinator,
guix-build-coordinator-queue-builds-configuration-systems,
guix-build-coordinator-queue-builds-configuration-system-and-targets,
guix-build-coordinator-queue-builds-configuration-guix-data-service,
guix-build-coordinator-queue-builds-configuration-processed-commits-file,
guix-build-coordinator-queue-builds-shepherd-services,
guix-build-coordinator-queue-builds-activation,
guix-build-coordinator-queue-builds-account): New procedures.
(guix-build-coordinator-queue-builds-service-type): New variable.
* doc/guix.texi (Guix Services): Document it.