cli: [FEAT] Add login command

[mdjunior]

This PR adds a login command using OpenID Connect flow with PKCE.

When executing the login command, the CLI client does a GET /status/config discovery call in the API and retrieves some information (Issuer URL and ClientID - named as oidc_base_url+oidc_realm and oidc_audience). With this information the CLI client has the information for an Authorization Code Flow with PKCE.
After this, the client starts a local web server (using a random port) and opens a web browser with OAuth 2.0 provider's consent page. After login in provider, the code argument is directed to the local server and the client terminates the flow.
The tokens are stored on OS keyring (refresh and access).

Help output:

$ gshc login -h

Initiates a new gsh session for a user. How authentication uses OpenID
Connect, it will open a web browser for the user to complete the login.

All gshc actions require the user to be authenticated (except [[gshc login]],
 [[gshc version]] and [[gshc target-*]]).

Usage:
  gshc login [flags]

Flags:
  -h, --help   help for login

Global Flags:
      --config string   config file (default is $HOME/.gshc.yaml)