[pkg/stanza] csv_input operator should support auto detection of header

[djaglowski]

CSV format allows headers to be specified as the first line in a file. Ideally, this library should support parsing of such files.

The csv_parser is only responsible for parsing one line of a file at a time, meaning that it cannot handle this basic use case on its own.

A csv_input operator could follow the example of the syslog_input operator, in that is wraps multiple operators together into a more useful form. In this case, it would wrap file_input and csv_parser. It should also provide a config flag such as first_line_headers to indicate whether or not the first line should be interpreted as headers.

[Dylan-M]

I would point out that it isn't always the first line that is the header. For example, from IIS 10, it is the 4th line, and it has a "#Fields:" prefix:

#Software: Microsoft Internet Information Services 10.0
#Version: 1.0
#Date: 2022-07-21 00:00:02
#Fields: date time s-ip cs-method cs-uri-stem cs-uri-query s-port cs-username c-ip cs(User-Agent) cs(Cookie) cs(Referer) cs-host sc-status sc-substatus sc-win32-status sc-bytes cs-bytes time-taken

[github-actions]

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