Support for structured logs

[kbence]

Logan should support JSON structured logs, where every line is a separate JSON object. Fields should be accessible through their names (or jq-like selectors for more complex cases).

E.g. in the following example:

{"time": 1234567890, "message": "msg", "userId": 54321}

fields should be accessible as $time, $message, $userId in filters and time, message, userId in field lists.

AST needs to be modified for that, maybe it's worth to first fix that issue.