regulators: Add Sensors service

Define the abstract base class Sensors. This class represents a service
that maintains a list of voltage regulator sensors.

This service makes t

regulators: Add Sensors service

Define the abstract base class Sensors. This class represents a service
that maintains a list of voltage regulator sensors.

This service makes the voltage regulator sensors available to other BMC
applications. For example, the Redfish support obtains sensor data from
this service.

Each voltage rail in the system may provide multiple types of sensor
data, such as temperature, output voltage, and output current. A sensor
tracks one of these data types for a voltage rail.

Also define a concrete subclass MockSensors used for test cases.

Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>
Change-Id: I0b1c28161bad899c6dd75f87b7ee8e891c8781af

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