refactor: move DbStructure.PgVersion to Config

Now that PgVersion is not part of DbStructure, Config is a more apt
module for it.

Also rename getDbStructure to queryDbStructure. AppState also had a
getDbStructure function for a record field.

src/PostgREST/Config/PgVersion.hs

@@ -0,0 +1,60 @@
+{-# LANGUAGE DeriveAnyClass #-}
+{-# LANGUAGE DeriveGeneric  #-}
+module PostgREST.Config.PgVersion
+  ( PgVersion(..)
+  , minimumPgVersion
+  , pgVersion95
+  , pgVersion96
+  , pgVersion100
+  , pgVersion109
+  , pgVersion110
+  , pgVersion112
+  , pgVersion114
+  , pgVersion121
+  , pgVersion130
+  ) where
+
+import qualified Data.Aeson as JSON
+
+import Protolude
+
+
+data PgVersion = PgVersion
+  { pgvNum  :: Int32
+  , pgvName :: Text
+  }
+  deriving (Eq, Generic, JSON.ToJSON)
+
+instance Ord PgVersion where
+  (PgVersion v1 _) `compare` (PgVersion v2 _) = v1 `compare` v2
+
+-- | Tells the minimum PostgreSQL version required by this version of PostgREST
+minimumPgVersion :: PgVersion
+minimumPgVersion = pgVersion95
+
+pgVersion95 :: PgVersion
+pgVersion95 = PgVersion 90500 "9.5"
+
+pgVersion96 :: PgVersion
+pgVersion96 = PgVersion 90600 "9.6"
+
+pgVersion100 :: PgVersion
+pgVersion100 = PgVersion 100000 "10"
+
+pgVersion109 :: PgVersion
+pgVersion109 = PgVersion 100009 "10.9"
+
+pgVersion110 :: PgVersion
+pgVersion110 = PgVersion 110000 "11.0"
+
+pgVersion112 :: PgVersion
+pgVersion112 = PgVersion 110002 "11.2"
+
+pgVersion114 :: PgVersion
+pgVersion114 = PgVersion 110004 "11.4"
+
+pgVersion121 :: PgVersion
+pgVersion121 = PgVersion 120001 "12.1"
+
+pgVersion130 :: PgVersion
+pgVersion130 = PgVersion 130000 "13.0"