OHM-449 Add function to get the extra mongo config
OHM-449 Add function to get the extra mongo config

This commit adds a function that returns a mongoose config object containing the extra configuration (readPreference, readConcern and writeConcern).

Structure of config object source: https://gist.github.com/taxilian/a05a5246b1d6a4c0bd2b26dfe5443d24#file-mongodb-d-ts-L289-L336

jembi/openhim-core-js,jembi/openhim-core-js