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.

testing/node_test_rig/Cargo.toml

@@ -10,7 +10,7 @@ beacon_node = { path = "../../beacon_node" }
 types = { path = "../../consensus/types" }
 eth2_config = { path = "../../common/eth2_config" }
 tempdir = "0.3.7"
-reqwest = "0.10.4"
+reqwest = { version = "0.10.4", features = ["native-tls-vendored"] }
 url = "2.1.1"
 serde = "1.0.110"
 futures = "0.3.5"