Properly separate whisper-client commands

[eabili0]

Today, there is no subcommands to the whisper-client cmd.

When executing ./whisper-client, the default behaviour is to execute its CheckCredentials subroutine.

We need it to have subcommands that corresponds to its available subroutines, namely:

1. Checkcredentials
2. IntrospectToken
3. GetOAuth2Client

[claudiosegala]

@abilioesteves what do you think about we divide ./whisper-client in these 3:

• ./whisper-client connect <all args> (CheckCredentials)
• ./whisper-client instrospect <token string> <whisper-url> (IntrospectToken)
• ./whisper-client get-client <args> (GetOAuth2Client)

[claudiosegala]

@abilioesteves another thing. What is supposed to return on the terminal the GetOAuth2Client?

[eabili0]

@claudiosegala, GetOAuth2Client's response is very specific for the golang's oauth2 default library. We should not expose it with the command-line interface, since it will only be used by other programming language stacks. Just connect and introspect is fine. Thanks!