Fix issue #708: Error handling, naming conventions, and commenting

This PR addresses the following issues in the GitLab Shell project:

1. Error return value of response.Body.Close is not checked in client.go
2. Struct field names in client.go and client_test.go do not follow the recommended naming conventions
3. Some exported types, functions and methods in client.go are not commented

The changes made in this PR include:

• Handling the error return value of response.Body.Close() in the VerifyOTP and PushAuth functions
• Renaming the struct fields in client.go and client_test.go to follow the recommended naming conventions
• Adding comments for the exported types, functions and methods in client.go

The changes have been verified by running the unit tests, which are now passing.