refactor: Make JWT authorization a middleware

This follows the style of wai-middleware-auth package and
makes the JWT parsing a middleware.

Co-authored-by: Wolfgang Walther <walther@technowledgy.de>

postgrest.cabal

@@ -108,6 +108,7 @@ library
                     , text                      >= 1.2.2 && < 1.3
                     , time                      >= 1.6 && < 1.11
                     , unordered-containers      >= 0.2.8 && < 0.3
+                    , vault                     >= 0.3.1.5 && < 0.4
                     , vector                    >= 0.11 && < 0.13
                     , wai                       >= 3.2.1 && < 3.3
                     , wai-cors                  >= 0.2.5 && < 0.3