Enable DEFAULT_API_KEYS via Docker Environment Variable

[CajuCLC]

Issue #, if available:
Issue 137: Pass DEFAULT_API_KEYS via Docker env

Description of changes:

• Modified api/settings.py to read DEFAULT_API_KEYS from the environment variable.
• Allows users to set DEFAULT_API_KEYS dynamically through Docker, providing enhanced flexibility for authentication configuration.

Example Docker command to run with custom API key:

docker run -d --name bedrock-gateway \
  -e AWS_ACCESS_KEY_ID=$AWS_ACCESS_KEY_ID \
  -e AWS_SECRET_ACCESS_KEY=$AWS_SECRET_ACCESS_KEY \
  -e AWS_SESSION_TOKEN=$AWS_SESSION_TOKEN \
  -e AWS_REGION=us-east-1 \
  -e DEFAULT_API_KEYS=your_custom_api_key \
  -p 8000:80 \
  bedrock-gateway

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