Add getA11yScore function to retrieve accessibility score from report categories

Author: max-ostapenko

definitions/functions.sqlx

@@ -0,0 +1,11 @@
+config {
+    type: 'operations'
+}
+
+CREATE FUNCTION IF NOT EXISTS `httparchive.fn.getA11yScore`(reportCategories JSON)
+RETURNS FLOAT64 DETERMINISTIC
+LANGUAGE js AS """
+if(reportCategories) {
+    return reportCategories.find(i => i.name === 'Accessibility').score;
+}
+""";