libethernet: Econet support

This patch series introduces preliminary Econet support through libapi_switchmgr library. It was made with the API/implementation decomposition in mind based on understanding that libethernet supports more functionality than just statistics retrieval.

The following features are implemented and tested:

• The API to get generic and rmon statistics is introduced.

There are minor changes in other libraries as well that are required for proper compilation of Econet target.

The functionality has undergone clang-tidy based analysis.