[Security solution] Elastic Assistant APIs - always use performChecks

[stephmilovic]

Summary

Makes changes around the performChecks function. Everywhere this was called was passing authenticatedUser: true and license: true, so I removed those parameters and updated the code so that those checks always happen. Previously, performChecks either returned an error response or undefined. I updated it to return either:

{
  isSuccess: true;
  currentUser: AuthenticatedUser;
} |
{
  isSuccess: false;
  response: IKibanaResponse;
};

That way when the check passes, it always returns our AuthenticatedUser and eliminates the need to re-run that check.

Finally, I found any spot that was checking license + user on its own to use the performChecks function.

[elasticmachine]

Pinging @elastic/security-solution (Team: SecuritySolution)

[stephmilovic]

Closing as these changes were included here #198239