input: Write a python function to encode a given string to its SHA-256 hash value.
  assert encode_string_sha256("hello") == "2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824"
  assert encode_string_sha256("world") == "486ea46224d1bb4fb680f34f7c9a1f9f6c50b179c8e1a1aa5a8df3d8d7b3fc2a"
  assert encode_string_sha256("test") == "9f86d081884c7d659a2feaa0c55ad015a3bf4f1b2b0b822cd15d6c15b0f00a08"
output: import hashlib
  
  def encode_string_sha256(s):
      return hashlib.sha256(s.encode()).hexdigest()