Add Llama 3 support to chat bot example

[mreso]

Description

This PR adds Llama3 support to the chat bot example (Llama 2 stays supported by changing model identifier)

Fixes #(issue)
#3099

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Feature/Issue validation/testing

Please describe the Unit or Integration tests that you ran to verify your changes and relevant result summary. Provide instructions so it can be reproduced.
Please also list any relevant details for your test configuration.

• docker run --rm -it --platform linux/amd64 -p 127.0.0.1:8080:8080 -p 127.0.0.1:8081:8081 -p 127.0.0.1:8082:8082 -p 127.0.0.1:8084:8084 -p 127.0.0.1:8085:8085 -v /Users/mreso/Projects/serve/model_store_1:/home/model-server/model-store -e MODEL_NAME=meta-llama/Meta-Llama-3-8B-Instruct pytorch/torchserve:chat_bot

Checklist:

• Did you have fun?
• Have you made corresponding changes to the documentation?

[agunapal]

LGTM