Switch webauthn-rs-demo to OpenSSL.

[micolous]

Fixes #359

This includes #357 (which makes rustls support optional behind --features tls), but switches everything over to tide-openssl and enables --features tls by default (because it works on aarch64-pc-windows-msvc and others).

Removing rustls and its transitive dependency on ring also drops the webauthn-rs-demo binary size a fair bit: a release build on x86_64-apple-darwin with dynamically-linked OpenSSL dropped the binary size from 9,444,928 bytes to 8,324,528 bytes (-12%); disabling tls support drops it further to 8,114,872 bytes (-14%).

This changes the behaviour of the server a fair bit – it won't generate a self-signed certificate at start-up anymore; it needs to be provided as a file.

I've updated the documentation to describe how TLS support works, and how to generate a self-signed certificate as the server used to generate.

This also updates the logging to use INFO level by default, so you actually see start-up messages now. :)

• cargo test has been run and passes
• documentation has been updated with relevant examples (if relevant)