Add support for more structured mlogfilter JSON output

[stennie]

mlogfilter --json output currently outputs a limited set of extracted fields with an array of split_tokens (using space as a delimiter) that do not have context.

More complete output of parsed log fields would be useful to convert logs for manipulation with jq or importing into other systems.

[stennie]

The output should align with MongoDB 4.4+ structured logging (aka logv2).

This would allow legacy logs to be roughly converted to the logv2 format for manipulation with newer tooling, although compatibility will generally be limited to field names rather than values.