MINOR: http_ext: add rfc7239_n2nn converter

Adding new http converter: rfc7239_n2nn.

Takes a string representing 7239 forwarded header node (extracted from
either 'for' or 'by' 7239 header fields) as input and translates it
to either ipv4 address, ipv6 address or str ('_' prefixed if obfuscated
or "unknown" if unknown), according to 7239RFC.

  Example:
    # extract 'for' field from forwarded header, extract nodename from
    # resulting node identifier and store the result in req.fnn
    http-request set-var(req.fnn) req.hdr(forwarded),rfc7239_field(for),rfc7239_n2nn
    #input: "for=\"127.0.0.1:9999\""
    #  output: 127.0.0.1
    #input: "for=\"_name:_port\""
    #  output: "_name"

Depends on:
  - "MINOR: http_ext: introduce http ext converters"
diff --git a/doc/configuration.txt b/doc/configuration.txt
index b0c64a5..37c2d33 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -17251,6 +17251,23 @@
     #input: "proto=https;host=\"haproxy.org:80\";for=\"127.0.0.1:9999\""
     #  output: "127.0.0.1:9999"
 
+rfc7239_n2nn
+  Converts RFC7239 node (provided by 'for' or 'by' 7239 header fields)
+  into its corresponding nodename final form:
+    - ipv4 address
+    - ipv6 address
+    - 'unknown'
+    - '_obfs' identifier
+
+  Example:
+    # extract 'for' field from forwarded header, extract nodename from
+    # resulting node identifier and store the result in req.fnn
+    http-request set-var(req.fnn) req.hdr(forwarded),rfc7239_field(for),rfc7239_n2nn
+    #input: "for=\"127.0.0.1:9999\""
+    #  output: 127.0.0.1
+    #input: "for=\"_name:_port\""
+    #  output: "_name"
+
 add(<value>)
   Adds <value> to the input value of type signed integer, and returns the
   result as a signed integer. <value> can be a numeric value or a variable