Add support to get channel lifecycle status

[deanna-lad]

Add support to get channel lifecycle status

Channel lifecycle status is described at [Channel status / REST / [Docs]|https://ably.com/docs/rest/channel-status#metadata-rest]

It is currently possible to retrieve channel lifecycle status in ably-go by using REST.Request and marshalling the result into a slice of empty interface.

Example:

result, _ := restClient.Request("get", "/channels/test").Pages(context.Background())

result.Next(ctx)

var items []interface{}
if err := result.Items(&items); err != nil {
	fmt.Println(err)
}

fmt.Printf("%+v\n", items)
// [map[channelId:test name:test status:map[isActive:true occupancy:map[metrics:map[connections:1 presenceConnections:1 presenceMembers:1 presenceSubscribers:1 publishers:1 subscribers:1]]]]]

Out of Scope
Realtime subscription to Channel Lifecycle Events. This ticket is only for supporting REST client ‘Get’.

Please see this /~https://github.com/ably/docs/pull/1415for more details

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