[sonic_y_cable] add abstract class YCableBase required for Y-cable AP…

…I support for multiple vendors (sonic-net#186)

Description
For multiple Y-Cable vendor support once we do have a mapping from vendor/part number to the appropriate Y-Cable module to load, we need to map appropriate port to a module as well. This PR adds definition for base abstract class.

Motivation and Context
Basically, the key idea is once we have a port identified as being of a certain vendor and it has been identified to load a certain module, it is then needed to call the correct module/API on each port each time we call the API on the port

it is required to maintain this mapping in memory since xcvrd does not want to read/parse this y_cable_vendor_mapping.py file again and again each time we call the Y-Cable API

Also note that the module loaded might change during xcvrd running lifetime since cable/transceiver can be changed from one vendor to another. So we need to take this into consideration as well

Proposed Solution for this
Each module of the Y-Cable vendor can be a class (of each transceiver type) and all we need to do is instantiate the objects of these classes as class instances and these objects will provide the interface of calling the API's for the appropriate vendor Y-Cable.

This instantiation will be done inside xcvrd, when xcvrd starts

These objects basically emulate Y-Cable instances and whatever action/interaction needs to be done with the Y-Cable the methods of these objects would provide that

each vendor in their implementation can inherit from a base class where there will be definitions for all the supported capabilities of the Y-Cable.

for vendors the recommended approach in case their subclass implementation does not support a method, is to set the method equal to None. This differentiates it from a method they forgot to implement. Then, the calling code should first check if the method is None before attempting to call it.

Design document for the support is sonic-net/SONiC#757

Signed-off-by: vaibhav-dahiya <vdahiya@microsoft.com>