fix URLsession leak in ServiceTokenProvider

[pbk20191]

URLsession does not dispose automatically when reference count is 0.
This is documented behavior.

class Foo: NSObject, URLSessionDelegate {
    
    func urlSession(_ session: URLSession, didBecomeInvalidWithError error: (any Error)?) {
        print("invalid session")
    }
    
    deinit {
        print("deinit")
    }
}
// nothing printed!!
do {
    let a : URLSession = URLSession(configuration: .default, delegate: Sooo(), delegateQueue: nil)
    let _ = a
}

So, creating URLSession every time to create Token cause URLSession to be leaked,
I would suggest to use default shared Session instead.

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