Add git commit verification input attributes

This implements the git input attributes `verifyCommit`, `keytype`,
`publicKey` and `publicKeys` as experimental feature
`verified-fetches`. `publicKeys` should be a json string.
This representation was chosen because all attributes must be of type bool,
int or string so they can be included in flake uris (see definition of
fetchers::Attr).

src/libfetchers/fetchers.cc

@@ -360,4 +360,9 @@ std::optional<ExperimentalFeature> InputScheme::experimentalFeature() const
     return {};
 }
 
+std::string publicKeys_to_string(const std::vector<PublicKey>& publicKeys)
+{
+    return ((nlohmann::json) publicKeys).dump();
+}
+
 }