fix: Dump media handlers and timezones with --dump-schema

Those were left out of the schema dump when the features were introduced, probably
because ByteString doesn't have a toJSON instance. Changing the type to Text solves
this easily.

Resolves #3237

src/PostgREST/SchemaCache/Routine.hs

@@ -110,7 +110,7 @@ data MediaHandler
    -- custom
    | CustomFunc QualifiedIdentifier RelIdentifier
    | NoAgg
-   deriving (Eq, Show)
+   deriving (Eq, Show, Generic, JSON.ToJSON)
 
 funcReturnsSingle :: Routine -> Bool
 funcReturnsSingle proc = case proc of