Add Selector.HasPending

[mfateev]

Is your feature request related to a problem? Please describe.
Currently to guarantee that every signal is delivered all channels have to be drained using ReadAsync before completing a workflow. This leads to code that looks like:

type channelConfig struct {
  channel workflow.ReceiveChannel
  val interface{}
  handler func()  // do something with val
}

channels := []*channelConfig{
  {
    channel: workflow.GetSignalChannel(...)
    val: "",
    handler func() {
      // do something with val
    },
  },
  // etc
}

s := workflow.NewSelector(ctx)
for _, cc := range channels {
  s.AddReceive(cc.channel, func (c workflow.ReceiveChannel, _ bool) {
    c.Receive(ctx, &cc.val)
    cc.handler()
  })
}

for keepGoing {
  s.Select(ctx)
  // not safe to return here
}

// Now I want to return but first I need to make sure that no signals are pending
canReturn := false
for !canReturn {
  canReturn = true
  for _, cc := range channels {
    if ok := c.ReceiveAsync(&cc.val); ok {
      canReturn = false
      cc.handler()
    }
  }
}

Describe the solution you'd like
Add Selector.HasPending method that returns true if Selector.Select is not going to block if called. This would simplify the above code to:

type channelConfig struct {
  channel workflow.ReceiveChannel
  val interface{}
  handler func()  // do something with val
}

channels := []*channelConfig{
  {
    channel: workflow.GetSignalChannel(...)
    val: "",
    handler func() {
      // do something with val
    },
  },
  // etc
}

s := workflow.NewSelector(ctx)
for _, cc := range channels {
  s.AddReceive(cc.channel, func (c workflow.ReceiveChannel, _ bool) {
    c.Receive(ctx, &cc.val)
    cc.handler()
  })
}

for keepGoing || s.HasPending {
  s.Select(ctx)
}

// safe complete workflow here

Describe alternatives you've considered
Add Channel.HasPending or ReceiveChannel.Peek. But it wouldn't help with Select and still require code complications.

Additional context
https://community.temporal.io/t/continueasnew-signals/1008/25