reminders.list api call implemented

[KarolisKl]

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Run make pr-prep from the root of the repository to run formatting, linting and tests. (passed)

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Description:

Implementation for https://api.slack.com/methods/reminders.list

Some changes in reminders.go:

• remindersResp struct for reminders.list response
• doReminders function for sending API request expecting reminders response mentioned above
• ListReminders function which calls the api request and returns reminders for the user
• One test added (since the call takes only the token parameter not much you can test there)

[kanata2]

LGTM👍