Add RawStatement support

RawStatement method creates new sql statements from raw query and optional map of named arguments.

postgres/statement.go

@@ -0,0 +1,8 @@
+package postgres
+
+import "github.com/go-jet/jet/v2/internal/jet"
+
+// RawStatement creates new sql statements from raw query and optional map of named arguments
+func RawStatement(rawQuery string, namedArguments ...RawArgs) Statement {
+	return jet.RawStatement(Dialect, rawQuery, namedArguments...)
+}