Mdm::Service text fields search

[MSP-10035](https://jira.tor.rapid7.com/secure/RapidBoard.jspa?rapidView=84&selectedIssue=MSP-10035)

Add search operators for `Mdm::Service#info` and `Mdm::Service#proto`.
Pushed a fix directly to master and bumped the version there to 0.19.1, but didn't release since it's just in a spec.  It can wait until the next release that will be consumed downstream.