Cross-compile to vendored x86_84, aarch64 (Raspberry Pi 4) (#1497)

## Issue Addressed

NA

## Proposed Changes

Adds support for using the [`cross`](https://github.com/rust-embedded/cross) project to produce cross-compiled binaries using Docker images.

Provides quite clean and simple cross-compiles cause all the complexity is hidden in Dockerfiles. It does require you to be in the `docker` group though.

## Details

- Adds shortcut commands to `Makefile`
- Ensures `reqwest` and `discv5` use vendored openssl libs (i.e., static not shared).
- Switches to a [commit](284f705964) of blst that has a renamed C function to avoid a collision with openssl (upstream issue: https://github.com/supranational/blst/issues/21).
- Updates `ring` to the latest satisfiable version, since an earlier version was causing issues with `cross`.
- Off-topic, but adds extra message about Windows support as suggested by Discord user.

## Additional Info

- ~~Blocked on #1495~~
- There are no tests in CI for this yet for a few reasons:
  - I'm hesitant to add more long-running tasks.
  - Short-term bitrot should be avoided since we'll use it each release.
  - In the long term I think it would be good to automate binary creation on a release.
- I observed the binaries increase in size from 50mb to 52mb after these changes.

book/src/pi.md

@@ -5,6 +5,11 @@ Tested on:
  - Raspberry Pi 4 Model B (4GB)
  - `Ubuntu 20.04 LTS (GNU/Linux 5.4.0-1011-raspi aarch64)`
 
+
+*Note: [Lighthouse supports cross-compiling](./cross-compiling.md) to target a
+Raspberry Pi (`aarch64`). Compiling on a faster machine (i.e., `x86_64`
+desktop) may be convenient.*
+
 ### 1. Install Ubuntu
 
 Follow the [Ubuntu Raspberry Pi installation instructions](https://ubuntu.com/download/raspberry-pi).