client.c test ECH support updates

[cpu]

Upstream we made some tweaks to the ech-client.rs example (rustls/rustls#2278) that motivates making a few changes to this repo's equivalent.

• The ech_fetch.rs util should be updated to write EchConfigListBytes files for each of the HTTPS records fetched, not just the first.
• The client.c test should be updated to accept a directory of ECH config list files in an env var instead of a single file.
• We might want to make some API changes to make it easier to filter many config lists into one working client config.

[cpu]

Fix is #504

The client.c test should be updated to accept a directory of ECH config list files in an env var instead of a single file.

I decided it was easier to make the env var a comma separated list of file paths.

We might want to make some API changes to make it easier to filter many config lists into one working client config.

It turns out the existing API is fine. It's pretty close to how you do this w/ the Rust API as well: try to convert each into an ECH mode. Stop for the first good one. Error if none are good.