input: Write a python function to create and validate JWT tokens using the Flask-JWT-Extended library.
  assert 'access_token' in create_and_validate_tokens('testuser', 'testpassword')
  assert 'refresh_token' in create_and_validate_tokens('testuser', 'testpassword')
  assert validate_token('testuser', 'access_token') == 'Welcome testuser'
  assert validate_token('wronguser', 'wrongtoken') == 'Invalid token or user'
output: from flask import Flask, jsonify
  from flask_jwt_extended import JWTManager, create_access_token, create_refresh_token, jwt_required, get_jwt_identity
  
  app = Flask(__name__)
  app.config['JWT_SECRET_KEY'] = 'your_secret_key'
  jwt = JWTManager(app)
  
  def create_and_validate_tokens(username, password):
      access_token = create_access_token(identity=username)
      refresh_token = create_refresh_token(identity=username)
      return {
          'access_token': access_token,
          'refresh_token': refresh_token
      }
  
  def validate_token(username, token):
      try:
          with app.test_request_context():
              decoded_token = jwt.decode_token(token)
              if decoded_token['identity'] == username:
                  return f'Welcome {username}'
      except Exception as e:
          return 'Invalid token or user'
      return 'Invalid token or user'