Remove support for credBlob, per #206

[micolous]

Per discussion on #206, this removes support for the credBlob extension:

• webauthn-authenticator-rs didn't implement it correctly (Fix type for AuthenticatorExtensionsClientOutputs::cred_blob, add related RegistrationExtensionsClientOutputs::cred_protect #206)
• I could only find two keys in FIDO's MDS which support this (ATKey.Pro and YubiKey Bio)
• The extension appears to be exclusively used for Microsoft's corporate Active Directory
• The User.id supports up to 64 bytes of arbitrary data and is supported by all CTAP2 authenticators (credBlob is only 32 bytes)
• The largeBlob extension supports even more data, and appears to replace it

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