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/libutil/experimental-features.hh

@@ -32,6 +32,7 @@ enum struct ExperimentalFeature
     ParseTomlTimestamps,
     ReadOnlyLocalStore,
     ConfigurableImpureEnv,
+    VerifiedFetches,
 };
 
 /**