Add new decoder for nginx basic auth

[eliasgrana]

Hi team,

Introduction

Currently, the dstuser field on Nginx basic authentication logs are not being decoded. Therefore, decoders are needed to extract if that information present.

Details

Logs collected have the following format:

2018/05/26 06:46:11 [error] 31963#31963: *28769 user "user1" was not found in "/etc/nginx/conf.d/users.htpasswd", client: 1.2.3.4, server: example.com, request: "GET / HTTP/1.1", host: "example.com"
2018/05/27 10:22:20 [error] 31972#31972: *52363 user "test user": password mismatch, client: 1.2.3.4, server: example.com, request: "GET / HTTP/2.0", host: "example.com"