Add readme for web provider

Also rename sample web provider implemented in Rust to `web-rust`.

providers/web/web-rust/src/utils.rs

@@ -0,0 +1,18 @@
+pub trait Filter<T> {
+    fn xfilter<P: FnOnce(&T) -> bool>(self, predicate: P) -> Self;
+}
+
+impl<T> Filter<T> for Option<T> {
+    fn xfilter<P: FnOnce(&T) -> bool>(self, predicate: P) -> Self {
+        match self {
+            Some(x) => {
+                if predicate(&x) {
+                    Some(x)
+                } else {
+                    None
+                }
+            }
+            None => None,
+        }
+    }
+}