CSV support for `file` source

[jszwedko]

User in discord was trying to figure out how to use remap to handle CSV files: https://discord.com/channels/742820443487993987/746070591097798688/821038938424344626

They are able to work with it by specifying the headers statically in their remap script like:

fields = split(.message, ",")
.SourceFile = fields[0]

But it seems like it'd be nice for the file source to support reading CSV files natively where it would generate events using the CSV header to name the fields of each line.

[fdamstra]

Coworker came up with a possible idea to handle CSVs (credit goes to him, not me for this idea and these words). We haven't tried this yet, but it seems like it could work for small files:

• Set multiline parsing to read the whole CSV file
• regex to the first newline. parse_csv the first line in .metadata as a .metadata.header
• split .message on newlines
• unnest .message on that split

The unnest will create new messages, one per line downstream with a .metadata.header and a .message, which can be further parsed/split and matched with the header values (how?).

Main limitation here is filesize as to do this it needs to store the whole CSV in memory, so that's probably a show stopper for many use cases. If anyone has ideas for improvements that would get around that then it may be a feasible idea.