Add chain element for selecting next element based on requested network service

[d-uzlov]

Overview

Sometimes we need have an endpoint that implements several services, and we want part of its chain to behave differently depending on the name of network service in the request. It would be very convenient to have a set of elements that each only work for one network service and choose one of them automatically.

Example usage:

choosebynetsvc.NewServer(
    map[string]networkservice.NetworkServiceServer{
        "service-1": point2pointipam.NewServer(ipnet1),
        "service-2": point2pointipam.NewServer(ipnet2),
        "service-3": point2pointipam.NewServer(ipnet3),
    }
)

[denis-tingaikin]

I like the idea of adding a chain element for multiservice endpoints.

I would be suggesting to change choosebynetsvc to selectservice or services by analogy with https://github.com/networkservicemesh/sdk/blob/main/pkg/networkservice/common/mechanisms/server.go#L21

[d-uzlov]

I like selectservice, I think it would be easier to read.