swbusd: support multiple endpoints for dual-stack

[yue-fred-gao]

What I did

• Support multiple endpoints in swbusd, such as one ipv4 and one ipv6.
• Use CancellationToken to gracefully shutdown the swbusd service. The same token can be cloned and shared by multiple services. On cancellation of the token, all the services are shutdown.
• Add dual-stack test case to test interconnect ipv4 and ipv6 clusters

Why I did it
It will be useful in ipv4/ipv6 mixed environment. For example, one cluster users ipv4 for swbusd connections and one cluster uses ipv6. To interconnect the 2 clusters, the gateway swbusd needs to support both endpoints.
It can work without dual endpoints to interconnect 2 clusters of different protocol. The limitation is the gateway from one cluster, e.g. IPv4, needs to define the peer (the gateway in IPv6 cluster) with its ipv6 endpoint.
How I verified it
Using the added test case.
Details if related

[mssonicbld]

/azp run