MSP-11905: add access_level, attempted_at, status attributes to credential export

The credential export (used in both Manage Credentials export as well as within the Zip Workspace export) was missing several attributes. These are included in export and import now, specs are updated to match, and old exports (without the new columns) are handled properly.

## Steps to verify
- [ ] On Pro master, in a workspace with logins (various statuses are desirable, run through these steps to get a good variety: https://github.com/rapid7/pro/pull/1657), generate a Zip Workspace export
- [ ] Change Pro Gemfile L48 to: ```gem 'metasploit-credential', :path => '/Users/YOU/rapid7/metasploit-credential'```
- [ ] ```bundle```
- [ ] Restart foreman, generate another Zip Workspace from the same workspace
- [ ] In one new workspace, import the first zip
- [ ] Verify you get no errors, verify credentials are added (all should have no access_level, Untried status)
- [ ] In another new workspace, import the second zip
- [ ] TODO