CTAP1/U2F commands should not be sent to devices that don't support it

[micolous]

Firefox bug: https://bugzilla.mozilla.org/show_bug.cgi?id=2063492

#276 added FidoDevice::downgrade_to_ctap1() as an infallible operation to work-around #269 (AppID extension pre-flighting).

However:

• a USB authenticator with the CAPABILITY_NMSG bit set will not accept CTAPHID_MSG (U2F) commands. It doesn't look like this library checks that flag.
• an NFC or smart card authenticator which responds to applet selection with FIDO_2_0 will not accept U2F commands.
• a BLE authenticator which sets fidoServiceRevisionBitfield = 0x20 does not support U2F commands.
• a caBLE/hybrid authenticator will never accept U2F commands, so this is required to support those.

In practice:

• Authenticators must disable CTAP1/U2F support when the Always Require User Verification feature is enabled.
• Yubikey (and likely others) allow disabling the U2F and FIDO2 applications individually.

FidoDevice::downgrade_to_ctap1() should be a fallible operation, and there should be a FidoDevice method which indicates whether a authenticator supports CTAP1 (ie: supports_ctap1()).

When use_ctap2_fallback = true, CTAP2-only authenticators should be excluded as a candidate device.

This issue is reproducible with Firefox Nightly on Linux.