Refactor db code. Add migration support

internal/repository/sql/query.sql

@@ -0,0 +1,25 @@
+-- name: GetAuthor :one
+SELECT * FROM authors
+WHERE id = ? LIMIT 1;
+
+-- name: ListAuthors :many
+SELECT * FROM authors
+ORDER BY name;
+
+-- name: CreateAuthor :one
+INSERT INTO authors (
+  name, bio
+) VALUES (
+  ?, ?
+)
+RETURNING *;
+
+-- name: UpdateAuthor :exec
+UPDATE authors
+set name = ?,
+bio = ?
+WHERE id = ?;
+
+-- name: DeleteAuthor :exec
+DELETE FROM authors
+WHERE id = ?;