We know that the BLE are dumb, they just send a bluetooth signals.
If we say we are using Estimote or Kontakt as a beacons provider, these two companies provide a mobile app to send the data received from the BLE beacon to the server.
I can use the google proximity api to register beacons, if i make a web application to do that or add beacons using postman, how the cloud will contact my beacon ? how can i know the data sent from this beacons if there is no middleware with the google apis and the beacon ?
The cloud does not contact your beacon and the beacons send no data.
The Google Proximity API requires that Google Play Services be installed on the phone. Such phones will periodically call out to Google servers to find out what beacons in the vicinity have associated data on Google's servers. Google Play Services will then respond appropriately to the Google Proximity API with the results.
All intelligence exists on the mobile phone and on Google's servers. The beacon remains a dumb transmitter sending just a unique identifier.