feat: add environment variable support for runtime interface emulator address

[cnaples79]

Summary

Add support for RUNTIME_INTERFACE_EMULATOR_ADDRESS environment variable to configure the emulator address without requiring CLI flags, addressing the need for easier configuration in containerized environments.

Motivation

Currently, the runtime interface emulator address can only be configured via the --runtime-interface-emulator-address CLI flag. In containerized environments where the entrypoint script is hardcoded (like in AWS Lambda base images), users must patch the Docker image to customize this address, which is cumbersome.

This change enables configuration via environment variable while maintaining backward compatibility and following the established pattern used by LOG_LEVEL.

Changes Made

• Environment Variable Support: Added RUNTIME_INTERFACE_EMULATOR_ADDRESS environment variable
• AWS CLI Precedence Pattern: CLI flag takes precedence over environment variable (consistent with AWS CLI behavior)
• Documentation Update: Updated help text to document the new environment variable
• Comprehensive Testing: Added integration tests for both environment variable usage and CLI precedence

Implementation Details

• Follows the exact same pattern as existing LOG_LEVEL environment variable handling
• Uses os.LookupEnv() to check for environment variable when CLI flag is at default value
• Maintains all existing validation and error handling
• Zero breaking changes - fully backward compatible

Usage Examples

# Set via environment variable
export RUNTIME_INTERFACE_EMULATOR_ADDRESS=127.0.0.1:9000
./aws-lambda-rie /lambda-entrypoint.sh handler

# CLI flag takes precedence (existing behavior)
./aws-lambda-rie --runtime-interface-emulator-address 127.0.0.1:9000 /lambda-entrypoint.sh handler

# Docker usage (addresses the original use case)
docker run -e RUNTIME_INTERFACE_EMULATOR_ADDRESS=0.0.0.0:8081 -p 8081:8081 my-lambda

Test Coverage

• ✅ Environment variable usage test
• ✅ CLI flag precedence test
• ✅ Backward compatibility (all existing tests pass)

Fixes #120

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Security Note: This change only affects local development/testing environments where the Lambda Runtime Interface Emulator is used. No production Lambda runtime changes.