add pkg login command and related functions

[girishramnani]

Description

Adds the ops pkg login <apikey> command which validates the api key from packagehub and stores it in ~/.ops/credentials to be used in other commands.

Testing

• Login to packagehub
• Generate an API key using the api key section
• use that api key in ops using ops pkg login <key>
• if the api key is correct you will see a message Login Successful as user <Username>.
• if the api key is incorrect you will see incorrect api-key error