[client] Move ICE and relay workers into the worker package

Relocate WorkerICE (renamed worker.ICE) and WorkerRelay plus ConnPriority
into client/internal/peer/worker. To break the peer<->worker cycle the
workers no longer take *Conn or ConnConfig: callbacks are passed as plain
functions (Conn's unexported methods as method values), and each worker
receives only the fields it needs (key, ICE config, isController) plus a
small services struct. Context is passed to OnNewOffer instead of stored.

Move the worker connection-status helper the other way, out of the worker
package into peer as worker_status.go (WorkerStatus / AtomicWorkerStatus),
since only Conn uses it.

client/internal/peer/worker/priority.go

@@ -0,0 +1,29 @@
+package worker
+
+import (
+	"fmt"
+)
+
+const (
+	None    ConnPriority = 0
+	Relay   ConnPriority = 1
+	ICETurn ConnPriority = 2
+	ICEP2P  ConnPriority = 3
+)
+
+type ConnPriority int
+
+func (cp ConnPriority) String() string {
+	switch cp {
+	case None:
+		return "None"
+	case Relay:
+		return "PriorityRelay"
+	case ICETurn:
+		return "PriorityICETurn"
+	case ICEP2P:
+		return "PriorityICEP2P"
+	default:
+		return fmt.Sprintf("ConnPriority(%d)", cp)
+	}
+}