Set OLAP Mode for mySQL

**Is your feature request related to a problem? Please describe.**
MySQL and Planetscale, as a result, impose query timeouts when running on OLTP mode. Setting the mode to OLAP via a SQL set query resolves this.

**Describe the solution you'd like**
Cube.js to support the setting of DB-level variables in the config file.

**Describe alternatives you've considered**
I spoke to Rory from cube cloud support, and we tried multiple types of pre-aggregations over days.

**Additional context**
Sample Error: (errno 2013) due to context deadline exceeded, elapsed time: 20.000298157s
What I need to execute: SET workload=olap;