macOS: use authentication information from git credential-osxkeychain

Created by: tillhainbach

Describe the feature or problem you'd like to solve

On macOS the default secure location to store sensible information (like tokens, passwords, etc.) is keychain. GitHub CLI and git both store authentication information there. This avoids saving tokens in plain text files.

Propose a Solution

Use git's credential-osxkeychain to set/get login credentials

Additional context

As far as I know git itself calls git credential-osxkeychain get host=the.host.com. This returns a string with format password=the_token\nusername=the_user which can be parsed to retrieve the need information for making authenticated api calls. I honestly have never written a single line of go, but if someone would point me to the right file (where the auth-info is read in) I might be able to create a draft PR.

Possibly related to #596 (closed)