Use new context for refreshing the key in background

[ejdem86grx]

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In its current shape the background refresh will never run, if the ctx passed to the go-jwks client will be canceled or expires before the library gets the new key.

Example code:

func External() {
  // 1. create a context with cancel
  ctx, cancel := context.WithCancel(context.Background())
  // 3. cancel is called before the method has finished, and this is causing the refreshKey method to fail because the context is cancelled at this point
  defer cancel()
  key, err := jwks.GetKey(ctx)
  // 2. the key is in the cache, but it should be refreshed, so GetKey starts a goroutine with the ctx passed to the refreshKey method
  return
}

[bigkraig]

@andresvia yep, it turned out to be a problem for us and we opened this PR