Add ability to configure CORS header (#1345)

## Issue Addressed

https://github.com/sigp/lighthouse/issues/1177

## Proposed Changes

Add a command line option (`--http-allow-origin`) and a config item for configuring the `Access-Control-Allow-Origin` response header.  This should unblock making XMLHttpRequests.

beacon_node/rest_api/src/config.rs

@@ -38,6 +38,9 @@ pub struct Config {
     pub listen_address: Ipv4Addr,
     /// The port the REST API HTTP server will listen on.
     pub port: u16,
+    /// If something else than "", a 'Access-Control-Allow-Origin' header will be present in
+    /// responses.  Put *, to allow any origin.
+    pub allow_origin: String,
 }
 
 impl Default for Config {
@@ -46,6 +49,7 @@ impl Default for Config {
             enabled: false,
             listen_address: Ipv4Addr::new(127, 0, 0, 1),
             port: 5052,
+            allow_origin: "".to_string(),
         }
     }
 }