GetAuthHandler gets the handler per the mode.

AuthModeBasic is basic mode.

AuthModeCustom is custom mode.

AuthModeNone means no auth required.

AuthModeOAuth is OAuth mode.

BaseHandler provides some basic functions like validation.

BasicAuthHandler handle the basic auth mode.

Credential stores the related data for authorization.

CustomAuthHandler handle the custom auth mode.

NoneAuthHandler handles the case of no credentail required.

TokenAuthHandler handles the OAuth auth mode.

Handler defines how to add authorization data to the requests depending on the different auth modes.