input: Write a function to create a web scraper that extracts specific data from a given website using BeautifulSoup. The function should take a URL as input, parse the webpage, and extract all the URLs in the 'href' attribute of 'a' tags. Return a list of these URLs.
  
  assert extract_urls("<url_of_webpage>")==["<url1>", "<url2>", "<url3>"]
  assert extract_urls("<another_url>")==["<urlA>", "<urlB>", "<urlC>"]
  assert extract_urls("<empty_url>")==[]
output: def extract_urls(url):
      import requests
      from bs4 import BeautifulSoup
      response = requests.get(url)
      soup = BeautifulSoup(response.text, 'html.parser')
      urls = [a['href'] for a in soup.find_all('a', href=True)]
      return urls