Add support for including custom metadata (HTTP headers) in method calls

[CCP-Zeulix]

The main use case being "authorization" e.g. via JWT.

import grpc
from your_protobuf_package import my_service_pb2_grpc, my_service_pb2

# Step 1: Create a channel to the server
channel = grpc.insecure_channel('localhost:50051')

# Step 2: Create the stub for the service
stub = my_service_pb2_grpc.MyServiceStub(channel)

# Step 3: Prepare the request object
request = my_service_pb2.MyRequest()

# Step 4: Create metadata with Authorization header
token = "your_jwt_token_here"
metadata = [("authorization", f"Bearer {token}")]

# Step 5: Make the gRPC call and pass metadata
try:
    response = stub.MyMethod(request, metadata=metadata)
    print("Response received:", response)
except grpc.RpcError as e:
    print(f"gRPC error: {e}")