Make the extending of REST layer more versatile

[jimirocks]

Currently, the REST layer is represented by spring RestTemplate which is created in package private method. This limits the extensionability of GoodData class (ie. by providing custom authentication or custom services). Moreover to provide custom REST layer one needs to introduce split-package problem preventing the library from future usage in java module system.

The redesigned solution should:

• Allow to supply custom REST layer, but at the same time reuse the default REST layer setup by extenders
• Allow to provide extenders custom services
• Allow to hookup custom authentication