[keyserver] Type ConnectionOptions passed to mysql2 library

Summary:
We should type this. (We use types from the mysql library because mysql2 is meant to be a drop-in replacement, and there is no Flow libdef for mysql2.)

Depends on D4515

Test Plan: Flow

Reviewers: jonringer-comm, atul, palys-swm

Reviewed By: atul

Subscribers: Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4529