services: nginx: Support extra content in the http block.

services: nginx: Support extra content in the http block.

This helpful when adding content to the nginx configuration file, which isn't
supported by the record type used for the configuration.  For example, like
adding proxy_cache_path configuration.

* gnu/packages/web.scm (<nginx-configuration>): Add new extra-content field.
  (nginx-configuration-extra-content): New field accessor.
  (default-nginx-config): Add support for the extra-content field.
* doc/guix.texi (NGINX): Document the new extra-content field.