Fix genesis state download panic when running in debug mode (#4753)

## Issue Addressed

#4738 

## Proposed Changes

See the above issue for details. Went with option #2 to use the async reqwest client in `Eth2NetworkConfig` and propagate the async-ness.

Dockerfile

@@ -1,4 +1,4 @@
-FROM rust:1.68.2-bullseye AS builder
+FROM rust:1.69.0-bullseye AS builder
 RUN apt-get update && apt-get -y upgrade && apt-get install -y cmake libclang-dev
 COPY . lighthouse
 ARG FEATURES