json: accept more characters in extract-prefix for json-parser()

[vincentbernat]

When receiving JSON objects whose keys have @ or - characters, it is
currently not possible to extract them with extract-prefix since they
are not considered as a valid JS identifier.

In JS, bracket notation should be used instead when accessing such
properties. This is not implemented in syslog-ng as only array indices
can be accessed with this notation.

This commit extends the set of supported characters to @ and -. -
can be easily found in the wild while @ is part of the event log
format of Logstash (@version and @timestamp).

This commit could be extended to support $ (which is a valid character
in a JS identifier) but this may conflict with variable expansion. Any
character other than [ and . could be accepted as they don't have a
meaning in the dot notation.

This PR can also be applied on previous versions of syslog-ng (at least 3.6).

[vincentbernat]

As an example, I am able to transform a v0 json file to a v1 version with this parser:

parser p_jsoneventv0 {
  # Transform to v1
  channel {
    parser { json-parser(extract-prefix("@fields")); };
    parser { json-parser(prefix(".json.")); };
    rewrite {
      set("1" value("@version"));
      set("${.json.@timestamp}" value("@timestamp"));
      set("${.json.@message}" value("message"));
    };
  };
};

[bazsi]

I also like this change :) And I am really happy to see such well thought out contributions. And with tests :)

[lbudai]

Squashing the commits is good idea ;-)

[vincentbernat]

Squashed.

[lbudai]

Thanks!!