Add support for app-required field state (#7166)

* Add "required" option to fields

* Move some exceptions to shared

* Do client side validation for required

* Add conditional required support

api/src/database/migrations/20210803A-add-required-to-fields.ts

@@ -0,0 +1,13 @@
+import { Knex } from 'knex';
+
+export async function up(knex: Knex): Promise<void> {
+	await knex.schema.alterTable('directus_fields', (table) => {
+		table.boolean('required').defaultTo(false);
+	});
+}
+
+export async function down(knex: Knex): Promise<void> {
+	await knex.schema.alterTable('directus_fields', (table) => {
+		table.dropColumn('required');
+	});
+}