Feature/msp 9641/cred rpc calls

This PR adds the RPC calls for new credential creation

VERIFICATION STEPS
- [ ] `msfconsole`
- [ ] `irb`
- [ ] `framework.db.workspace.id` Remember this value!
- [ ] `exit`
- [ ] `load msgrpc Pass=pass123`
- [ ] VERIFY the rpc service starts up
- [ ] in another terminal `rvmsudo ./msfrpc -P pass123 -a 127.0.0.1 -S`
- [ ] when the rpc terminal opens, paste in the following code (replace the workspace id with the value you got above)
```
credential_data = {
    module_fullname: 'auxiliary/scanner/afp/afp_login',
    origin_type: :service,
    private_data: 'msfadmin',
    private_type: :password,
    username: 'msfadmin',
    last_attempted_at: DateTime.now.to_s,
    status: 'Successful',
    address: '99.99.99.99',
    port: 548,
    service_name: 'afp',
    protocol: 'tcp',
    workspace_id: 60
}

rpc.call('db.create_credential', credential_data)
```
- [ ] VERIFY you get a hash back with details about the credential that was created
- [ ] run `cred` in msfconsole
- [ ] VERIFY you see the credential listed