Add pin_protocol field to HmacSecretInput

The December 08, 2020 version of the Client to Authenticator Protocol
(CTAP) specification added the pinUvAuthProtocol(0x04) field to the
input data for the hmac-secret extension in the
authenticatorGetAssertion command.  This patch adds this new field to
the HmacSecretInput struct to make it possible to parse requests using
this new field.
	https://fidoalliance.org/specs/fido-v2.1-rd-20201208/fido-client-to-authenticator-protocol-v2.1-rd-20201208.html

Fixes #1.

src/ctap2/get_assertion.rs

@@ -21,7 +21,8 @@ pub struct HmacSecretInput {
     // *either* enc(salt1) *or* enc(salt1 || salt2)
     pub salt_enc: Bytes<64>,
     pub salt_auth: Bytes<16>,
-
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub pin_protocol: Option<u32>
 }
 
 #[derive(Clone,Debug, Eq,PartialEq,Serialize,Deserialize)]