Allow configuring indexes for JSON fields

Problem/Motivation

For now, all the log entry data in the database is stored as a single JSON field, which allows querying not only by substring, but by specific values too. But for a large amount of logs, the performance of such queries will not be very good, because concrete values in the JSON fields are not indexed.

Proposed resolution

Would be good to extend the module to allow configuring indexes for specific JSON values using JSONPath and database virtual fields.