Add SQL Server cache configuration to appsettings.json

Added a "SqlServer" object under the "Cache" section in
`appsettings.json` with properties for "ConnectionString",
"SchemaName" (set to "dbo"), and "TableName" (set to "TBDD_CACHE").
No changes were made to the "SignatureCacheExpiration" property,
but it was re-added in the diff for context.

EnvelopeGenerator.API/appsettings.json

@@ -175,7 +175,12 @@
     "EmailTemplate": [ "TBSIG_EMAIL_TEMPLATE_AFT_UPD" ]
   },
   "Cache": {
-    "SignatureCacheExpiration": null
+    "SignatureCacheExpiration": null,
+    "SqlServer": {
+      "ConnectionString": null,
+      "SchemaName": "dbo",
+      "TableName": "TBDD_CACHE"
+    }
   },
   "MainPageTitle": null,
   "AnnotationParams": {