connect-tests: add EchConfig fetch tests

This commit adds a new `connect-tests/tests/ech.rs` module that performs
a DNS over HTTPS lookup for HTTPS type records, finding `EchConfig`s and
testing we can deserialize the raw form into the Rustls representation
without error.

Presently it tests against:
* `crypto.cloudflare.com`
* `defo.ie`
* `tls-ech.dev`

Since these are network based tests they need to live in `connect-tests`
to avoid flakyness during normal CI runs.

In previous WIP branches this was done as part of an overall end-to-end
example of using ECH, but we can test this in isolation ahead of having
full ECH support.