Allow to run custom SQL queries

[kean]

With Pulse 2.0, the storage is now fully SQL-compatible, and it's no longer feasible to surface all the possible filters in the UI. While, as a user, you could open the logs in a database viewer and query it, there are major advantages of views the data in the Pulse console. Pulse should support running custom SQL queries and there should be a couple of examples of such queries in the documentation.