CI performance improvements

[micolous]

This does a few things to speed up CI tasks:

• build in debug mode (rather than release mode), so the tests don't trigger a full rebuild
• only build the many features with webauthn-authenticator-rs, rather than the whole library
• split cargo fmt into its own action
• don't run clippy for webauthn-authenticator-rs on Windows unless using Windows-specific features
• use sccache

There's more jobs now, but they're smaller jobs, so less likely to block things up when you have a few branches running in parallel.

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