Update to masked credential format

https://jira6.osdc.lax.rapid7.com/browse/MSP-9219

See Sam before reviewing.

## Steps to verify
- [x] Have a host with a service
- [x] Add a credential to that service, password type
- [x] Exports -> Export Data
- [x] Generate an XML format export with defaults
- [x] Verify the export file generated has the password in plain text
- [x] Generate an XML format export, check the Mask Credentials option
- [x] Verify the export file generated has `*MASKED*` for the private part, should not show password in plaintext
- [x] In a new workspace, import the first PWDump file
- [x] Verify the host is added with a service and a credential with the correct password
- [x] Delete the host or in a new workspace: import the second PWDump file
- [x] Verify the host and service are added, but not the credential


This is a small change to the format of masked creds assumed by the importer to match changes in export/report refactoring.