Improve timeout handling

[fako1024]

Right now, there is no default client-side timeout enforced (also refer to this quite commonly known discussion), and the WithTimeout() method only sets a context-based timeout (which will also take effect in case a request just takes a while, like downloading a large file).
While it's possible to specify a custom Transport using WithTransport, allowing the caller to set their own timeouts, there should probably be a) better defaults and b) some convenience methods to cover some common scenarios easily.