Support pre-flight CORS check (#1772)

## Issue Addressed

- Resolves #1766 

## Proposed Changes

- Use the `warp::filters::cors` filter instead of our work-around.

## Additional Info

It's not trivial to enable/disable `cors` using `warp`, since using `routes.with(cors)` changes the type of `routes`.  This makes it difficult to apply/not apply cors at runtime. My solution has been to *always* use the `warp::filters::cors` wrapper but when cors should be disabled, just pass the HTTP server listen address as the only permissible origin.

beacon_node/http_api/Cargo.toml

@@ -5,7 +5,7 @@ authors = ["Paul Hauner <paul@paulhauner.com>"]
 edition = "2018"
 
 [dependencies]
-warp = "0.2.5"
+warp = { git = "https://github.com/paulhauner/warp", branch = "cors-wildcard" }
 serde = { version = "1.0.116", features = ["derive"] }
 tokio = { version = "0.2.22", features = ["macros"] }
 parking_lot = "0.11.0"