Proposal: app-port watch mode

## Describe the proposal

Current CLI is not easy to develop Dapr user application because it always needs to have user app executable. The problem is that a developer always needs to build the binary for the testing and it is hard to debug user app from IDE's debuggers without attaching debugger to process. It results in the low productivity in order to develop user app.

In order to resolve this problem, this proposes `app-port watch mode` to cli.

For example,

```
dapr watch --app-id mynode --app-port 3000 --port 3500
```

1. dapr cli will monitor `localhost:3000` port. cli will not run daprd unless 3000 port is available.
2. When 3000 port is available, Dapr cli runs daprd and then daprd connects to user app.
3. cli will monitor the port on the background and if the port is unavailable, cli will kill daprd process