services: guix: Add guix-build-coordinator-agent-service-type.
* gnu/services/guix.scm (<guix-build-coordinator-agent-configuration>): New
record type.
(guix-build-coordinator-agent-configuration,
guix-build-coordinator-agent-configuration?,
guix-build-coordinator-agent-configuration-package,
guix-build-coordinator-agent-configuration-user,
guix-build-coordinator-agent-configuration-coordinator,
guix-build-coordinator-agent-configuration-uuid),
guix-build-coordinator-agent-configuration-password,
guix-build-coordinator-agent-configuration-password-file,
guix-build-coordinator-agent-configuration-systems,
guix-build-coordinator-agent-configuration-max-parallel-builds,
guix-build-coordinator-agent-configuration-derivation-substitute-urls,
guix-build-coordinator-agent-configuration-non-derivation-substitute-urls,
guix-build-coordinator-agent-shepherd-services,
guix-build-coordinator-agent-activation,
guix-build-coordinator-agent-account): New procedures.
(guix-build-coordinator-agent-service-type): New variable.
* doc/guix.texi (Guix Services): Document it.