Add DefaultTenantAccessor and ITenantAccessor interface

Introduce ITenantAccessor interface and its implementation, DefaultTenantAccessor, for tenant management in the multitenancy module. This setup will allow future enhancements for tenant-specific functionality.

src/modules/Elsa.Common/Multitenancy/Contracts/ITenantAccessor.cs

@@ -0,0 +1,10 @@
+namespace Elsa.Common.Multitenancy;
+
+public interface ITenantAccessor
+{
+    /// <summary>
+    /// Get the current <see cref="Tenant"/>.
+    /// </summary>
+    /// <returns>Current tenant or null.</returns>
+    Tenant? GetTenant();
+}

src/modules/Elsa.Common/Multitenancy/Implementations/DefaultTenantAccessor.cs

@@ -0,0 +1,13 @@
+namespace Elsa.Common.Multitenancy;
+
+public class DefaultTenantAccessor : ITenantAccessor
+{
+    public Tenant? GetTenant()
+    {
+        // Similar to how the HttpContextAccessor is implemented:
+        
+        return null;
+        
+        
+    }
+}