ingest-user-agent: provide a way to extend the default regexes file

[AE777F63]

Describe the feature:
Currently in order to add new entries to the regexes file one needs to copy the default file, add the new entries, and configure the pipeline to use the new file. It would be convenient to be able to add new entries without having to maintain the content of the default file.
For example, regex_file option could be extended to accept an array of paths, i.e.

      "user_agent" : {
        "field" : "agent",
        "regex_file": ["custom-prepend.yml", "_default_", "custom-append.yml"]
      }

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Ingest)