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/ReadPlan.hs

@@ -7,14 +7,14 @@ module PostgREST.Plan.ReadPlan
 
 import Data.Tree (Tree (..))
 
-import PostgREST.DbStructure.Identifiers  (FieldName,
-                                           QualifiedIdentifier)
-import PostgREST.DbStructure.Relationship (Relationship)
-import PostgREST.RangeQuery               (NonnegRange)
-import PostgREST.Request.Types            (Alias, Depth, Hint,
+import PostgREST.ApiRequest.Types         (Alias, Depth, Hint,
                                            JoinCondition, JoinType,
                                            LogicTree, NodeName,
                                            OrderTerm, SelectItem)
+import PostgREST.RangeQuery               (NonnegRange)
+import PostgREST.SchemaCache.Identifiers  (FieldName,
+                                           QualifiedIdentifier)
+import PostgREST.SchemaCache.Relationship (Relationship)
 
 
 import Protolude