What is the average rating of all the podcasts with reviews created in 2019?
created in 2019 refers to created_at BETWEEN '2019-01-01T00:00:00' and '2019-12-31T23:59:59'; average rating  = Divide (Sum(rating), Count(podcast_id))
SELECT AVG(rating) FROM reviews WHERE created_at BETWEEN '2019-01-01T00:00:00-07:00' AND '2019-12-31T23:59:59-07:00'
database name:db_id music_platform_2
runs table runs Cols: run at dtype text, max row id dtype integer, reviews added dtype integer, 
podcasts table podcasts Cols: podcast id dtype text, itunes id dtype integer, slug dtype text, itunes url dtype text, title dtype text, 
reviews table reviews Cols: podcast id dtype text, title dtype text, content dtype text, rating dtype integer, author id dtype text, created at dtype text, 
categories table categories Cols: podcast id dtype text, category dtype text, 
