Remove use of ARTRealtimePresence query in public API

See comment; the problem described there arose when Marat tried to
implement a mock of Presence for #4.

Sources/AblyChat/Presence.swift

@@ -5,7 +5,7 @@ public typealias PresenceData = any Sendable
 
 public protocol Presence: AnyObject, Sendable, EmitsDiscontinuities {
     func get() async throws -> [PresenceMember]
-    func get(params: ARTRealtimePresenceQuery?) async throws -> [PresenceMember]
+    func get(params: PresenceQuery?) async throws -> [PresenceMember]
     func isUserPresent(clientID: String) async throws -> Bool
     func enter() async throws
     func enter(data: PresenceData) async throws
@@ -61,3 +61,24 @@ public struct PresenceEvent: Sendable {
         self.data = data
     }
 }
+
+// This is a Sendable equivalent of ably-cocoa’s ARTRealtimePresenceQuery type.
+//
+// Originally, ``Presence.get(params:)`` accepted an ARTRealtimePresenceQuery object, but I’ve changed it to accept this type, because else when you try and write an actor that implements ``Presence``, you get a compiler error like "Non-sendable type 'ARTRealtimePresenceQuery' in parameter of the protocol requirement satisfied by actor-isolated instance method 'get(params:)' cannot cross actor boundary; this is an error in the Swift 6 language mode".
+//
+// Now, based on my limited understanding, you _should_ be able to send non-Sendable values from one isolation domain to another (the purpose of the "region-based isolation" and "`sending` parameters" features added in Swift 6), but to get this to work I had to mark ``Presence`` as requiring conformance to the `Actor` protocol, and since I didn’t understand _why_ I had to do that, I didn’t want to put it in the public API.
+//
+// So, for now, let’s just accept this copy (which I don’t think is a big problem anyway); we can always revisit it with more Swift concurrency knowledge in the future. Created https://github.com/ably-labs/ably-chat-swift/issues/64 to revisit.
+public struct PresenceQuery: Sendable {
+    public var limit = 100
+    public var clientID: String?
+    public var connectionID: String?
+    public var waitForSync = true
+
+    internal init(limit: Int = 100, clientID: String? = nil, connectionID: String? = nil, waitForSync: Bool = true) {
+        self.limit = limit
+        self.clientID = clientID
+        self.connectionID = connectionID
+        self.waitForSync = waitForSync
+    }
+}