Improve the type of PresenceMember.extras

The type that I gave it in 20e7f5f was just copied from the JS SDK, but
we can do better - make it strongly typed, using a type that’s
consistent with PresenceData.

Part of #13.

Sources/AblyChat/DefaultPresence.swift

@@ -268,8 +268,11 @@ internal final class DefaultPresence: Presence, EmitsDiscontinuities {
                 throw error
             }
 
-            // Seems like we want to just forward on `extras` from the cocoa SDK but that is an `ARTJsonCompatible` type which is not `Sendable`... currently just converting this to a `Sendable` type (`String`) until we know what to do with this.
-            let extras = member.extras?.toJSONString()
+            let extras: [String: JSONValue]? = if let ablyCocoaExtras = member.extras {
+                JSONValue.objectFromAblyCocoaExtras(ablyCocoaExtras)
+            } else {
+                nil
+            }
 
             let presenceMember = PresenceMember(
                 clientID: clientID,

Sources/AblyChat/Presence.swift

@@ -166,7 +166,7 @@ public struct PresenceMember: Sendable {
         }
     }
 
-    public init(clientID: String, data: PresenceData?, action: PresenceMember.Action, extras: (any Sendable)?, updatedAt: Date) {
+    public init(clientID: String, data: PresenceData?, action: PresenceMember.Action, extras: [String: JSONValue]?, updatedAt: Date) {
         self.clientID = clientID
         self.data = data
         self.action = action
@@ -195,7 +195,7 @@ public struct PresenceMember: Sendable {
     /**
      * The extras associated with the presence member.
      */
-    public var extras: (any Sendable)?
+    public var extras: [String: JSONValue]?
     public var updatedAt: Date
 }