Metasploit::Credential::Core association search

[MSP-10029](https://jira.tor.rapid7.com/secure/RapidBoard.jspa?rapidView=84&view=detail&selectedIssue=MSP-10029)

Search logins, private, public, and realm.  Searching tags is left to a Pro PR because Tags for credentials are only implemented in Pro in anticipation that `Mdm::Tag`s will be moved to Pro.