Fix/SK-1178 | Set client_id in ModelRequest + single hearbeat function

[Wrede]

Short

• Use client_id for the client id instead of setting "name" to be the id.
• Add single hearbeat function for easier testing.

Below are the key changes:

gRPC Connection Enhancements:

• Added keepalive settings and gRPC options to ensure the connection remains open for long-lived clients (fedn/network/clients/grpc_handler.py).

Refactoring:

• Refactored the _init_secure_channel and _init_insecure_channel methods to use the new GRPC_OPTIONS for connection settings (fedn/network/clients/grpc_handler.py). [1] [2]
• Extracted the heartbeat logic into a separate heartbeat method and updated the send_heartbeats method to utilize this new method (fedn/network/clients/grpc_handler.py).

Bug Fixes:

• Corrected the attribute name from name to client_id in get_model_from_combiner and Download methods to accurately reflect the sender's client ID (fedn/network/clients/grpc_handler.py, fedn/network/combiner/modelservice.py). [1] [2]