services: guix-publish: Allow for multi-compression.

This is a followup to b8fa86ad.

* guix/deprecation.scm (warn-about-deprecation): Make public.
* gnu/services/base.scm (<guix-publish-configuration>)[compression]: New
field.
[compression-level]: Default to #f.  Add '%' to getter name.
(guix-publish-configuration-compression-level): Define as deprecated.
(default-compression): New procedure.
(guix-publish-shepherd-service)[config->compression-options]: New
procedure.
Use 'match-record' instead of 'match'.
* doc/guix.texi (Base Services): Remove 'compression-level' and document
'compression'.