fix copyfrom methods on interface not matching generated methods (#1424)

Author: alexdulin

internal/codegen/golang/templates/pgx/interfaceCode.tmpl

@@ -28,9 +28,9 @@
             {{.MethodName}}(ctx context.Context, {{.Arg.Pair}}) (pgconn.CommandTag, error)
         {{- end}}
         {{- if and (eq .Cmd ":copyfrom") ($dbtxParam) }}
-            {{.MethodName}}(ctx context.Context, db DBTX, {{.Arg.Pair}}) (int64, error)
+            {{.MethodName}}(ctx context.Context, db DBTX, {{.Arg.SlicePair}}) (int64, error)
         {{- else if eq .Cmd ":copyfrom" }}
-            {{.MethodName}}(ctx context.Context, {{.Arg.Pair}}) (int64, error)
+            {{.MethodName}}(ctx context.Context, {{.Arg.SlicePair}}) (int64, error)
         {{- end}}
     {{- end}}
     }

internal/endtoend/testdata/copyfrom/postgresql/pgx/go/querier.go

@@ -7,7 +7,8 @@
       "sql_package": "pgx/v4",
       "name": "querytest",
       "schema": "query.sql",
-      "queries": "query.sql"
+      "queries": "query.sql",
+      "emit_interface": true
     }
   ]
 }