OpenSearch output module: add CLI option to pass predefined output field

[joachimmetz]

use case: when exporting the contents of a Plaso storage file, add a custom (user defined) field to OpenSearch output.

Idea add custom fields option e.g. --custom_fields="hostname:MYHOST"

[joachimmetz]

Note to self

• this might be a good/more elegant alternative to the current log2timeline.py --text (text prepend) option
• what if a custom field is set that overlaps with a "regular" field? for now give the "regular" field precedence

[DfirJos]

Yes, this would be perfect! I'm also looking for ways to include the hostname. I have a not-so-elegant solution which is uploading the file from a Path which includes the hostname - this way I can see the hostname in Opensearch. Let me know if you found a more elegant solution. In the meantime I will track this enhancement.