Add support for string message in Bedrock textgen

[jonminkin97]

Description

Enable llm-textgen using Bedrock to accept a string message as well as a list of chat messages.

Issues

n/a

Type of change

List the type of change like below. Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Dependencies

n/a

Tests

Build and run opea/llm-textgen container. Verify that existing chat format and new string message format both work.

# Existing functionality
curl http://${host_ip}:9009/v1/chat/completions \
  -X POST \
  -d '{"model": "us.anthropic.claude-3-5-haiku-20241022-v1:0", "messages": [{"role": "user", "content": "What is Deep Learning?"}], "max_tokens":17}' \
  -H 'Content-Type: application/json'

# New functionality
curl http://${host_ip}:9009/v1/chat/completions \
  -X POST \
  -d '{"model": "us.anthropic.claude-3-5-haiku-20241022-v1:0", "messages": "What is Deep Learning?", "max_tokens":17}' \
  -H 'Content-Type: application/json'