Fix UV-skipping logic with required and preferred

[micolous]

Fixes #373, matching the behaviour defined in the CTAP and WebAuthn specs:

• Don't skip UV when uv = required.

  Previously, UV would be skipped when a device did not have some form of UV configured, and required the RPs to reject the request (which they need to do anyway for security reasons).

• Don't skip UV when uv = preferred and an authenticator supports any form of user verification.

  Previously, this would be skipped when a device did not have some form of UV configured.

  UV is still skipped for authenticators that don't support any form of user verification (CTAP 1, and some CTAP 2.0 devices). Supporting UV is mandatory in CTAP 2.1 and later, but this doesn't enforce those rules.

  Unfortunately, this change will cause an authenticator that supports UV but is unconfigured to now fail those operations in Firefox, because it doesn't prompt to set PINs at enrolment time. These would previously proceed without UV.

• Refactor the can_skip_user_verification checks to make them closer to CTAP 2.3's validation steps.

• Update docs for AuthenticatorOptions::user_verification to use CTAP 2.3's wording, noting caveats with CTAP 2.0 and reformat it to look nicer in rustdoc.

Testing

I tested this with the ctap2.rs example. All changes in bold marked with ⭐. Any authenticator options not specified are missing.

Authenticator	MC required	MC preferred	MC discouraged	GA required	GA preferred	GA discouraged
CTAP 2.1, clientPin, alwaysUv, !makeCredUvNotRqd	required	required	required	required	required	required
CTAP 2.1, clientPin, !alwaysUv, makeCredUvNotRqd	required	required	no UV	required	required	no UV
CTAP 2.1, !clientPin, !alwaysUv,makeCredUvNotRqd	fails ⭐	fails ⭐	no UV	fails ⭐	fails ⭐	no UV
CTAP 2.1-PRE, clientPin	required	required	required	required	required	no UV
CTAP 2.1-PRE, !clientPin	fails ⭐	fails ⭐	no UV	fails ⭐	fails ⭐	no UV
CTAP 2.0, clientPin	required	required	required	required	required	no UV
CTAP 2.0, !clientPin	fails ⭐	fails ⭐	no UV	fails ⭐	fails ⭐	no UV
CTAP 1/U2F	fails	no UV	no UV	fails	no UV	no UV

[micolous]

CI failure looks unrelated, opened #375 for that.