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"
}
]
}
```
