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.

doc/guix.texi

@@ -15938,6 +15938,10 @@ use the size of the processors cache line.
 @item @code{server-names-hash-bucket-max-size} (default: @code{#f})
 Maximum bucket size for the server names hash tables.
 
+@item @code{extra-content} (default: @code{""})
+Extra content for the @code{http} block.  Should be string or a string
+valued G-expression.
+
 @end table
 @end deffn
 

gnu/services/web.scm

@@ -77,6 +77,7 @@
             nginx-configuration-upstream-blocks
             nginx-configuration-server-names-hash-bucket-size
             nginx-configuration-server-names-hash-bucket-max-size
+            nginx-configuration-extra-content
             nginx-configuration-file
 
             <nginx-server-configuration>
@@ -431,6 +432,8 @@
                                  (default #f))
   (server-names-hash-bucket-max-size nginx-configuration-server-names-hash-bucket-max-size
                                      (default #f))
+  (extra-content nginx-configuration-extra-content
+                 (default ""))
   (file          nginx-configuration-file         ;#f | string | file-like
                  (default #f)))
 
@@ -521,7 +524,8 @@ of index files."
                 (nginx log-directory run-directory
                  server-blocks upstream-blocks
                  server-names-hash-bucket-size
-                 server-names-hash-bucket-max-size)
+                 server-names-hash-bucket-max-size
+                 extra-content)
    (apply mixed-text-file "nginx.conf"
           (flatten
            "user nginx nginx;\n"
@@ -550,7 +554,8 @@ of index files."
            "\n"
            (map emit-nginx-upstream-config upstream-blocks)
            (map emit-nginx-server-config server-blocks)
-           "}\n"
+           extra-content
+           "\n}\n"
            "events {}\n"))))
 
 (define %nginx-accounts