[FR] FireLens - add support for extra parser files

[JohnPreston]

Currently, in Advanced, I can add SourceFile which allow me to add filters etc, which will be included to the final configuration that FireLens will generate.

But if I have custom Parser, I need to set a ParserFile property pointing to it, and cannot have it as part of the main configuration file. Therefore, I'd like to have

x-logging:
  FireLens:
    Advanced:
      SourceFile: <>
      ParserFiles:
        - /path/to/parser

ParserFiles would be a list of files to add to the shared docker volume and add to the general [SERVICE] instruction.