Add mapping for regconfig type

Fixes #516

(cherry picked from commit 76da2f3)

Author: roji

src/EFCore.PG/Storage/Internal/Mapping/NpgsqlRegconfigTypeMapping.cs

@@ -0,0 +1,22 @@
+using System.Text;
+using Microsoft.EntityFrameworkCore.Storage;
+using Microsoft.EntityFrameworkCore.Storage.ValueConversion;
+using Npgsql.EntityFrameworkCore.PostgreSQL.Utilities;
+using NpgsqlTypes;
+
+namespace Npgsql.EntityFrameworkCore.PostgreSQL.Storage.Internal.Mapping
+{
+    public class NpgsqlRegconfigTypeMapping : NpgsqlTypeMapping
+    {
+        public NpgsqlRegconfigTypeMapping() : base("regconfig", typeof(string), NpgsqlDbType.Unknown) { }
+
+        protected NpgsqlRegconfigTypeMapping(RelationalTypeMappingParameters parameters, NpgsqlDbType npgsqlDbType)
+            : base(parameters, npgsqlDbType) {}
+
+        public override RelationalTypeMapping Clone(string storeType, int? size)
+            => new NpgsqlRegconfigTypeMapping(Parameters.WithStoreTypeAndSize(storeType, size), NpgsqlDbType);
+
+        public override CoreTypeMapping Clone(ValueConverter converter)
+            => new NpgsqlRegconfigTypeMapping(Parameters.WithComposedConverter(converter), NpgsqlDbType);
+    }
+}

src/EFCore.PG/Storage/Internal/NpgsqlTypeMappingSource.cs

@@ -107,6 +107,7 @@ public class NpgsqlTypeMappingSource : RelationalTypeMappingSource
         // Full text search mappings
         readonly NpgsqlTsQueryTypeMapping   _tsquery               = new NpgsqlTsQueryTypeMapping();
         readonly NpgsqlTsVectorTypeMapping  _tsvector              = new NpgsqlTsVectorTypeMapping();
+        readonly NpgsqlRegconfigTypeMapping _regconfig             = new NpgsqlRegconfigTypeMapping();
         readonly NpgsqlTsRankingNormalizationTypeMapping _rankingNormalization = new NpgsqlTsRankingNormalizationTypeMapping();
 
         // Built-in ranges
@@ -214,7 +215,8 @@ public NpgsqlTypeMappingSource([NotNull] TypeMappingSourceDependencies dependenc
                 { "daterange",                   new[] { _daterange                    } },
 
                 { "tsquery",                     new[] { _tsquery                      } },
-                { "tsvector",                    new[] { _tsvector                     } }
+                { "tsvector",                    new[] { _tsvector                     } },
+                { "regconfig",                   new[] { _regconfig                    } }
             };
 
             var clrTypeMappings = new Dictionary<Type, RelationalTypeMapping>