Think about to make use of functional options

## Context

Many API endpoints (or even the "NewClient" function) make use of optional options.
Those are very often a struct. If you don't require options, you pass nil.

An alternative would be https://dave.cheney.net/2014/10/17/functional-options-for-friendly-apis

## Goal of this task

* Think about this solution, if this is feasible
* Start a prototype / proof of concept

This request comes from https://github.com/andygrunwald/go-jira/issues/200.
An own ticket was created to keep the development contained and focus the discussion.