Introduce Sub Channel ID Node to entities to identify the internal channel of a channel ID

[MattB-msft]

Within the concept of Clients/Channel providers there are some providers that group a set of channels behind a single "public" channel.
There are currently two examples of this inside the MSFT ecosystem.

• Azure Communications Services
• M365 msTeams Channel

In these cases, multiple clients are fronted by a single service,

Currently, the Activity Protocol described a single channel concept called "channel Id".
To handle the updated requirements, we would introduce a new component in entitles called "subChannelId".
This subchannel would be used to describe the subchannel being fronted

This would take the shape of:

            "subChannelInfo": {
                "channelId": "COPILOT"
            }

Which would existing in the structure:

{
    "id": "1741193418370",
    "channelId": "msteams",
    "entities": [
        {
            "subChannelInfo": {
                "channelId": "COPILOT"
            }
        }
    ],
    "channelData": {
        "tenant": {
            "id": "000000-0000-0000-00000-00000000000"
        },
        "productContext": "COPILOT"
    }
}