Enhance Form Submission Export Functionality (#657)

* Enhance Form Submission Export Functionality

* Validate new param 'columns'

* Form submission export request as seprate class with validation

* Test case for export

---------

Co-authored-by: Julien Nahum <julien@nahum.net>

client/components/open/forms/components/FormSubmissions.vue

@@ -213,7 +213,7 @@ export default {
       if (!this.form) {
         return ''
       }
-      return this.runtimeConfig.public.apiBase + '/open/forms/' + this.form.id + '/submissions/export'
+      return this.runtimeConfig.public.apiBase + 'open/forms/' + this.form.id + '/submissions/export'
     },
     isLoading() {
       return this.recordStore.loading
@@ -333,8 +333,13 @@ export default {
         return
       }
       this.exportLoading = true
-      opnFetch(this.exportUrl, {responseType: "blob"})
-        .then(blob => {
+      opnFetch(this.exportUrl, {
+        responseType: "blob",
+        method: "POST",
+        body: {
+          columns: this.displayColumns
+        }
+      }).then(blob => {
           const filename = `${this.form.slug}-${Date.now()}-submissions.csv`
           const a = document.createElement("a")
           document.body.appendChild(a)