Implement presence API for IServiceConnectionContainer

[Y-Sindo]

To expose presence API to the caller in the IServiceConnectionContainer interface, we have two choices:

1. Expose a low-layer method like "WriteXXXMessage", which accepts a GroupMemberQueryMessage parameter and return GroupMemberQueryResponse. However, the MultipleEndpointServiceConnectionContainer also implements this interface and has to implement this method. In multi-endpoint case, we have to have some aggregation logic on the results from all the endpoints. Then the verb Write cannot describe what has been done in multiple endpoint cases. Therefore we don't expose a low-layer method.
2. Expose a high-layer method ListConnectionsInGroup, to wrap operations including writing messages, fetch next pages according to continuationToken.

[vwxyzh]

rough idea for n-way merge for multi-endpoint.
#2126

sequenceDiagram
    participant c as Client
    participant s as SDK
    participant m as Multi Endpoint Service Connection Container
    participant s1 as Service Connection Container For Service 1
    participant s2 as Service Connection Container For Service 2

    c->>s: Enumerate group member
    s->>m: Enumerate group member
    loop
        opt Need more data from Service 1
            m->>s1: Enumerate group member with pagination
            s1->>m: Pagination result
        end
        opt Need more data from Service 2
            m->>s2: Enumerate group member with pagination
            s2->>m: Pagination result
        end
        loop
            m->>m: N-way merge
            m->>s: Yield return
            s->>c: Yield return
        end
    end

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