This is a redo of #54
I ported it manually to remove unrelated conflicting changes and avoid conflicts. The bulk of the code is the initial implementation from @jacquelinekay in #54.

The current prototype passes the tests for all rmw implementation except connext_dynamic (due to lack of Nested messages and service support).

What remains to do:

• move utilities to operate on rcl_interfaces C data structures to the rcl_interfaces package
• modify the current client/service functions to take a type support and thus be type-erased
  • remove internal storage for requests and responses from internal parameter_client
• address any interface changes that will come with feedback from Parameters refactor ros2#432
• enforce valid parameter names and "namespaces"
• polish and finish convenience functions

Bonus levels:

• provide a way for user to register parameter validation functions
• check/enforce uniqueness of parameter name
• provide more utilities / convenience functions for C array management
• add parameter array types
  ...

Connects to #28