Import REST Apis from the current BBSim implementation

Third parties have implemented workflows around BBSim REST Apis, we should maintain the same features and the same interface.

Note that is ok to have a different "primary" gRPC interface as long as there is support to convert call with the older interface into the primary one. The rationale behind this is that the new redesign leverage state machine to keep track of the device state in the workflow and it's important for the operator to easily query that state.

The proposal it to implement the existing APIs under the "/legacy" prefix.

(Please comment to start a discussion if this cause any concern)

For example:

GET /v1/olt
message Olt {
    int32 ID = 1;
    string OperState = 2;
    string InternalState = 3;
    repeated NNIPort NNIPorts = 4;
    repeated PONPort PONPorts = 5;
}

GET /legacy/olt [2]
message OLTStatusResponse {
    OLTInfo olt = 1;
    repeated PortInfo ports = 2;
}

1. message OLT:  https://github.com/opencord/bbsim/blob/master/api/bbsim/bbsim.proto#L28-L34
2. message OLTStatusResponse: https://github.com/opencord/voltha-bbsim/blob/master/api/bbsim.proto#L89-L92

Note that the new BBSim API is not stable yet, so the Olt message is subject to changes.