add basic ARCHITECTURE.md (#2503)

* refactor: move ApiRequest a top-level module
* refactor: rename DbStructure to SchemaCache
* refactor: GucHeader inside Response
* refactor: admin app to Workers

src/PostgREST/Plan/CallPlan.hs

@@ -5,9 +5,9 @@ module PostgREST.Plan.CallPlan
 where
 
 import qualified Data.ByteString.Lazy              as LBS
-import           PostgREST.DbStructure.Identifiers (FieldName,
+import           PostgREST.SchemaCache.Identifiers (FieldName,
                                                     QualifiedIdentifier)
-import           PostgREST.DbStructure.Proc        (ProcParam (..))
+import           PostgREST.SchemaCache.Proc        (ProcParam (..))
 
 import Protolude