input: Write a python function to handle a GET request and return a welcome message.
  assert lambda_handler({'httpMethod': 'GET'}, None) == {
      'statusCode': 200,
      'headers': {'Content-Type': 'application/json'},
      'body': json.dumps({'message': 'Welcome!'})
  }
  assert lambda_handler({'httpMethod': 'POST'}, None) == None
output: import json
  
  def lambda_handler(event, context):
      if event['httpMethod'] == 'GET':
          return {
              'statusCode': 200,
              'headers': {'Content-Type': 'application/json'},
              'body': json.dumps({'message': 'Welcome!'})
          }
      return None