Generate valid structure for MakeCredential params

* Add crate arbitrary as ctap's optional dependency, when feature "fuzz"
  is activated.

* Derive Arbitrary for all the necessary types in order to generate the
  concrete types from random bytes.

* Add a fuzz target that transforms the input to valid format for
  MakeCredential.

fuzz/fuzz_targets/fuzz_target_process_ctap2_make_credential_structured.rs

@@ -0,0 +1,10 @@
+#![no_main]
+
+use fuzz_helper::{process_ctap_structured, InputType};
+use libfuzzer_sys::fuzz_target;
+
+// Fuzz inputs as CTAP2 make credential command parameters.
+// The inputs will used to construct arbitrary make credential parameters.
+fuzz_target!(|data: &[u8]| {
+    process_ctap_structured(data, InputType::CborMakeCredentialParameter).ok();
+});