If BidirectionalStream request contains a -network-isolation-key
header, it is used to set the network isolation key of the stream.
The header itself is removed and not transmitted.
The header value should be a valid URL with different host and port
for each different network isolation key. Invalid header value is
reported by returning error from bidirectional_stream_start.
Network isolation takes effect only if it is enabled by experimental
option of
"feature_list": {
"enable-features": "PartitionConnectionsByNetworkIsolationKey"
}
