Refactor SQL execution classes into new namespace

Restructure and refactor classes related to SQL execution within the `EnvelopeGenerator.Infrastructure` namespace. Key changes include:
- Added `EnvelopeGenerator.Infrastructure.Executor` namespace.
- Moved and redefined `Query`, `QueryExtension`, `SQLExecutor`, `SQLExecutorBaseEntity`, and `SQLExecutorParams` classes to the new namespace.
- Maintained existing functionality while improving code organization and clarity.

EnvelopeGenerator.Infrastructure/Executor/SQLExecutorParams.cs

@@ -0,0 +1,6 @@
+namespace EnvelopeGenerator.Infrastructure.Executor;
+
+public class SQLExecutorParams
+{
+    public string? ConnectionString { get; set; }
+}