[pulsar-perf] Enable pulsar-perf to load WebSocket service URL from conf file

[massakam]

Motivation

Currently, the command line option --proxy-url is required when running the subcommand websocket-producer of pulsar-perf.

$ ./bin/pulsar-perf websocket-producer

The following option is required: -u, --proxy-url
Usage: pulsar-perf websocket-producer [options] persistent://tenant/ns/my-topic
  Options:
    ...
  * -u, --proxy-url
       Pulsar Proxy URL, e.g., "ws://localhost:8080/"

I think it would be useful to be able to load the WebSocket service URL from the config file (such as client.conf) as well as other parameters.

Modifications

pulsar-perf loads the WebSocket service URL from the config file if --proxy-url is not specified. The priorities are as follows:

1. --proxy-url option
2. webSocketServiceUrl in the config file
3. webServiceUrl in the config file
4. serviceUrl in the config file