build: Add feature switches for conditional vendoring of dependencies

[CyberTailor]

Is your feature request related to a problem? Please describe.

webauthn-rs-core crate is not comptaible with LibreSSL (see kanidm/webauthn-rs#402) and fails to build against it. OpenSSL 1.1 is also unsupported.

Describe the solution you'd like

Add a feature switch to depend on openssl/vendored.

Describe alternatives you've considered

Fix build with LibreSSL or make WebAuthn functionality optional (controlled with a feature switch).

Additional context

A feature switch controlling vendoring libsqlite3-sys could also be added.

[Firstyear]

You already can build this with cargo build --features openssl/vendored (but if I recall, vendored openssl is still 1.1 for some reason).