The class NearbyService supports the discovery of services provided by
nearby devices and supports communicating with those services through
message-based data, streaming data, and resources (such as files).
In iOS, the framework uses infrastructure Wi-Fi networks, peer-to-peer Wi-Fi,
and Bluetooth personal area networks for the underlying transport.
param serviceType max length 15 character, need to be defined in Info.plist
Begins advertising the service provided by a local peer.
The startAdvertisingPeer publishes an advertisement for a specific service
that your app provides through the flutter_nearby_connections plugin and
notifies its delegate about invitations from nearby peers.
Starts browsing for peers.
Searches (by serviceType) for services offered by nearby devices using
infrastructure Wi-Fi, peer-to-peer Wi-Fi, and Bluetooth or Ethernet, and
provides the ability to easily invite those Device to a earby connections
session SessionState.
stateChangedSubscription helps you listen to the changes of peers with
the circumstances: find a new peer, a peer is invited, a peer is disconnected,
a peer is invited to connect by another peer, or 2 peers are connected.
stateChangedSubscription will return you a list of Device.
see StateChangedCallback