Validator API (#504)

* Implemented more REST API endpoints.
 - Added many of the endpoints, which return 501 - Not Implemented
 - Created helper function to return a not implemented error
 - Created a parse_pubkey function to get a PublicKey from a hex string
 - Created a HTTP handler for the validator endpoints
 - Started implementing validator/duties endpoint.

* Fleshed out get validator duties.
 - Re-implemented the get validator duties function for the REST API
 - Added an 'as_hex_string' function to FakePublicKey, beacuse it was missing.

* Fixed small caching/state bug.

* Extended to array of API inputs.
 - Created  function for getting arrays from GET parameters.
 - Extended get validator duties function to support array of validator duties.

* Tidy API to be more consistent with recent decisions

* Addressing Paul's comments.
 - Cleaning up function to get list of proposers.
 - Removing unnecessary serde annotations
 - Clarifying error messages
 - Only accept pubkeys if they are '0x' prefixed.

* Fixed formatting with rustfmt.

beacon_node/rest_api/src/url_query.rs

@@ -2,6 +2,8 @@ use crate::ApiError;
 use hyper::Request;
 
 /// Provides handy functions for parsing the query parameters of a URL.
+
+#[derive(Clone, Copy)]
 pub struct UrlQuery<'a>(url::form_urlencoded::Parse<'a>);
 
 impl<'a> UrlQuery<'a> {
@@ -11,9 +13,7 @@ impl<'a> UrlQuery<'a> {
     pub fn from_request<T>(req: &'a Request<T>) -> Result<Self, ApiError> {
         let query_str = req.uri().query().ok_or_else(|| {
             ApiError::InvalidQueryParams(
-                "URL query must be valid and contain at least one
-                                         key."
-                    .to_string(),
+                "URL query must be valid and contain at least one key.".to_string(),
             )
         })?;
 
@@ -60,6 +60,23 @@ impl<'a> UrlQuery<'a> {
             )))
         }
     }
+
+    /// Returns a vector of all values present where `key` is in `keys
+    ///
+    /// If no match is found, an `InvalidQueryParams` error is returned.
+    pub fn all_of(mut self, key: &str) -> Result<Vec<String>, ApiError> {
+        let queries: Vec<_> = self
+            .0
+            .filter_map(|(k, v)| {
+                if k.eq(key) {
+                    Some(v.into_owned())
+                } else {
+                    None
+                }
+            })
+            .collect();
+        Ok(queries)
+    }
 }
 
 #[cfg(test)]