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.

beacon_node/eth2_libp2p/Cargo.toml

@@ -32,7 +32,7 @@ snap = "1.0.0"
 void = "1.0.2"
 tokio-io-timeout = "0.4.0"
 tokio-util = { version = "0.3.1", features = ["codec", "compat"] }
-discv5 = { version = "0.1.0-alpha.8", features = ["libp2p"] }
+discv5 = { version = "0.1.0-alpha.8", features = ["libp2p", "openssl-vendored"] }
 tiny-keccak = "2.0.2"
 environment = { path = "../../lighthouse/environment" }
 # TODO: Remove rand crate for mainnet