Make external_id a required var in webhook-datasource module (#26)

Change summary:
----------------
The external_id variable was made optional so as to not break
onboarding backend. In the backend, we always ensure passing
the external_id to the module, because if not provided, the
validation for the same would fail via WIF.

Making this required now (as it should be) since backend is all merged.

modules/services/webhook-datasource/README.md

@@ -70,7 +70,7 @@ No modules.
 | <a name="input_project_id"></a> [project\_id](#input\_project\_id) | (Required) Target Project identifier provided by the customer | `string` | n/a | yes |
 | <a name="input_push_endpoint"></a> [push\_endpoint](#input\_push\_endpoint) | (Required) Final endpoint towards which audit logs POST calls will be directed | `string` | n/a | yes |
 | <a name="input_role_name"></a> [role\_name](#input\_role\_name) | (Optional) Role name for custom role binding to the service account, with read permissions for data ingestion resources | `string` | `"SysdigIngestionAuthRole"` | no |
-| <a name="input_external_id"></a> [external\_id](#input\_external\_id) | (Optional) Random string generated unique to a customer | `string` | `""` | no |
+| <a name="input_external_id"></a> [external\_id](#input\_external\_id) | (Required) Random string generated unique to a customer | `string` | n/a | yes |
 | <a name="input_suffix"></a> [suffix](#input\_suffix) | (Optional) Suffix to uniquely identify resources during multiple installs. If not provided, random value is autogenerated | `string` | `null` | no |
 
 ## Outputs

modules/services/webhook-datasource/variables.tf

@@ -66,8 +66,7 @@ variable "role_name" {
 
 variable "external_id" {
   type        = string
-  description = "Random string generated unique to a customer"
-  default     = ""
+  description = "(Required) Random string generated unique to a customer"
 }
 
 variable "suffix" {