Expose HTTP client parameters, make it configurable

[tock-ibm]

In session.go, creation of an http client has many hard coded constants. Expose those and make it configurable with good defaults.

func newHTTPClient() *http.Client {
	httpClient := &http.Client{
		Transport: &http.Transport{
			Proxy: http.ProxyFromEnvironment,
			DialContext: (&net.Dialer{
				Timeout:   30 * time.Second,
				KeepAlive: 30 * time.Second,
			}).DialContext,
			ForceAttemptHTTP2:     true,
			MaxIdleConns:          100,
			MaxIdleConnsPerHost:   100,
			IdleConnTimeout:       90 * time.Second,
			TLSHandshakeTimeout:   10 * time.Second,
			ExpectContinueTimeout: 1 * time.Second,
		},
	}
	return httpClient
}