Allow custom HttpClient to NewClient

[JoseAlban]

When creating a new client e.g.:

osocloud.NewClient(oso.LocalURL, oso.LocalAuthToken)

I can't find any way to set up a custom timeout, e.g. when running tests, I wanted this to fail quick, like 1 second, but can only rely on the long default timeout, which is annoying :(

Ideally: we can pass a custom HttpClient.
Even better, use the functional options pattern for every other option too