services: guix-data-service: Allow passing extra options.

services: guix-data-service: Allow passing extra options.

This is so that the options supported by the service configuration don't have
to always be changed. Generally though all options should be explicitly
supported and documented, so this is mostly to facilitate experimentation.

* gnu/services/guix.scm (<guix-data-service-configuration>): Add extra-options
and extra-process-jobs-options to the record type.
(guix-data-service-shepherd-services): Handle these new configuration record
fields.
* doc/guix.texi (Guix Data Service): Document these new options.