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.

Cargo.lock

@@ -521,7 +521,7 @@ dependencies = [
 [[package]]
 name = "blst"
 version = "0.1.1"
-source = "git+https://github.com/sigp/blst.git?rev=22bfb91721af125d1cb08aa201a18477665a45fe#22bfb91721af125d1cb08aa201a18477665a45fe"
+source = "git+https://github.com/sigp/blst.git?rev=284f7059642851c760a09fb1708bcb59c7ca323c#284f7059642851c760a09fb1708bcb59c7ca323c"
 dependencies = [
  "cc",
  "glob",
@@ -3468,6 +3468,15 @@ version = "0.1.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "77af24da69f9d9341038eba93a073b1fdaaa1b788221b00a69bce9e762cb32de"
 
+[[package]]
+name = "openssl-src"
+version = "111.10.2+1.1.1g"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a287fdb22e32b5b60624d4a5a7a02dbe82777f730ec0dbc42a0554326fef5a70"
+dependencies = [
+ "cc",
+]
+
 [[package]]
 name = "openssl-sys"
 version = "0.9.58"
@@ -3477,6 +3486,7 @@ dependencies = [
  "autocfg 1.0.0",
  "cc",
  "libc",
+ "openssl-src",
  "pkg-config",
  "vcpkg",
 ]