add server side validation for emails (#8022)

related to https://github.com/fleetdm/fleet/issues/7199, this adds email validation to the `verifyCreateShared` which is used for user creation in the server.

validation messages come directly from Go's `net/mail` package.

```
~/fleet $ curl 'https://localhost:8080/api/latest/fleet/users/admin' -X POST -H 'Authorization: Bearer $TOKEN' --data-raw '{"email":"asdf","name":"asdf@asd.com","password":"as;lkdfjasdlk;fja3234@","global_role":"observer","teams":[]}'
{
  "message": "Validation Failed",
  "errors": [
    {
      "name": "email",
      "reason": "mail: missing '@' or angle-addr"
    }
  ]
}
```

changes/7199-email-validation

@@ -0,0 +1 @@
+* Added server-side validation of user emails