Introduce ClientParameters for OkHttp support
This commit introduces ClientParameters to gather common parameters and
avoid adding new constructors for every new parameter. It also
introduces the RestTemplateConfigurator extension point to customize the
RestTemplate used in the Client. This allows among others choosing the
request factory used in the RestTemplate. The support for Apache
HTTP Components client has been refactored to fit into this design, same
thing for OkHttp.
The ClientSpec test has been refactored to use Spock "where" blocks to
test both Apache HTTP Components and OkHttp implementations.