case insensitive support for regex in elastic_ecs

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Is your feature request related to a problem? Please describe.
Currently, the elastic_ecs connector does not support case insensitive flag like /abc/i or /(?i)abc/, and hunters need to write command_line MATCHES '([Vv][Ii][Rr][Uu][Ss])'.

Describe the solution you'd like
From syntax, we expect users to write command_line MATCHES '(?i)virus'. After checking more into Elasticsearch docs, we found the most feasible solution to do it in stix-shifter is to unfold (?i) in translation and keep the transmission as current implementation.