wip: test ECH w/ client.c

This is a work-in-progress for a handful of reasons:

1. It's not clear what the best way to get a hold of ECH configs to test
   with...

You need to get these from DNS, but ideally using encrypted
DNS-over-HTTPS or you're leaking the servername before the TLS
handshake.

`dig` supports this but only with a very recent version which will be
annoying (especially on Windows...). We'll also need to decode the
BASE64 to get the raw encoded form.

We can't easily vendor a copy of a config because these configs tend to
be rotated on a fairly frequent basis.

The approach I've used for now is to add a test binary that fetches
the raw configs for a hardcoded test host using HickoryDNS. This also
kind of sucks since a dev-dep on Hickory is fairly large (and without
care, breaks our MSRV). Ugh!

2. There's some kind of bug to sort out where some requests to the
   host w/ ECH works, but some connections are returning data
   without a content length header and breaking the loop we use to test
   multiple requests.

Need to debug this :-) It's probably something minor? (fingers-crossed).

With the two caveats above in mind, there is some signs of life:

```
RUSTLS_ECH_CONFIG_LIST=tests/research.cloudflare.com.ech.configs.der \
  RUSTLS_PLATFORM_VERIFIER=1 \
  ./target/client research.cloudflare.com 443 /cdn-cgi/trace 2>/dev/null \
    | grep "sni="

sni=encrypted
sni=encrypted
sni=encrypted

```