Metasploit::Credential::Core association search

[MSP-10029](https://jira.tor.rapid7.com/secure/RapidBoard.jspa?rapidView=84&view=detail&selectedIssue=MSP-10029)

Use `metasploit-credential` that supports searching associations on `Metasploit::Credential::Core` for https://github.com/rapid7/pro compatibility.