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.

crypto/bls/Cargo.toml

@@ -17,7 +17,7 @@ eth2_hashing = "0.1.0"
 ethereum-types = "0.9.1"
 arbitrary = { version = "0.4.4", features = ["derive"], optional = true }
 zeroize = { version = "1.0.0", features = ["zeroize_derive"] }
-blst = { git = "https://github.com/sigp/blst.git", rev = "22bfb91721af125d1cb08aa201a18477665a45fe" }
+blst = { git = "https://github.com/sigp/blst.git", rev = "284f7059642851c760a09fb1708bcb59c7ca323c" }
 
 [features]
 default = ["supranational"]

crypto/eth2_hashing/Cargo.toml

@@ -10,7 +10,7 @@ description = "Hashing primitives used in Ethereum 2.0"
 lazy_static = { version = "1.4.0", optional = true }
 
 [target.'cfg(not(target_arch = "wasm32"))'.dependencies]
-ring = "0.16.9"
+ring = "0.16.12"
 
 [target.'cfg(target_arch = "wasm32")'.dependencies]
 sha2 = "0.9.1"