Add an alternative locust client

[YukioZzz]

I. What this PR does

Add locust client as an alternative, new features are as follows:

• simpler test file (locustfile.py) that just defines how to call the server API
• both gRPC and HTTP supported
• load test params could be specified in yaml
• prometheus metrics export supported

II. Why we need it

As we might want the client to perform a more customized load test to the inference server in a more simple and flexible way, an idea is to use locust as the backend so that the user can directly specify load test params or provide its own locusttest script. A demo client image based on locust is provided here.

[lwangbm]

1. [pkg/client_locust/image.jpg] if this is used for demo, pls rmv it to the demo folder
2. [pkg/client_locust] pls provide docs(e.g., readme) and demo here, showing how to use the new feature
3. the pull request tittle: Add an alternative locust client. pls provide detailed description for this pull request: the motivation, new features provided, etc

[YukioZzz]

1. [pkg/client_locust/image.jpg] if this is used for demo, pls rmv it to the demo folder
2. [pkg/client_locust] pls provide docs(e.g., readme) and demo here, showing how to use the new feature
3. the pull request tittle: Add an alternative locust client. pls provide detailed description for this pull request: the motivation, new features provided, etc

1. I am not sure whether should I remove [pkg/client_locust/image.jpg]. It's just like the file [pkg/client/image.jpg] and will be used by the demo client.
2. README.md is added now, new features are as follows:

• simpler test file needed
• both gRPC and HTTP supported
• load test params could be specified in yaml
• prometheus metrics export supported

3. I got it, changing it now
   let me know what I should further change then~ thanks for your patience.