Add the bilateral toggle support for the basic SSS model

Assets/ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/Resources/CombineSubsurfaceScattering.shader

                     [flatten]
                     if (any(sampleIrradiance))
                     {
+                    #if (SSS_BILATERAL != 0)
                         // Apply bilateral weighting.
                         // Ref #1: Skin Rendering by Pseudo–Separable Cross Bilateral Filtering.
                         // Ref #2: Separable SSS, Supplementary Materials, Section E.
                         sampleWeight     *= exp(-zDistance * zDistance * halfRcpVariance);
+                    #endif
 
                         totalIrradiance += sampleWeight * sampleIrradiance;
                         totalWeight     += sampleWeight;