feat(crypto): Add support for Schnorr auxiliary inputs

spec/_attachments/ic.did

@@ -322,10 +322,17 @@ type schnorr_public_key_result = record {
     chain_code : blob;
 };
 
+type schnorr_aux = variant {
+    bip341: record {
+      merkle_root_hash: blob;
+   }
+};
+
 type sign_with_schnorr_args = record {
     message : blob;
     derivation_path : vec blob;
     key_id : record { algorithm : schnorr_algorithm; name : text };
+    aux: opt schnorr_aux;
 };
 
 type sign_with_schnorr_result = record {

spec/index.md

@@ -2447,6 +2447,8 @@ The encoding of the signature depends on the key ID's `algorithm`:
 
 This call requires that a Schnorr key with ID `key_id` was generated by the IC and the signing functionality for that key was enabled. Otherwise, the call is is rejected.
 
+This call accepts an optional auxiliary parameter `aux`. The auxiliary parameter type `schnorr_aux` is a variant. The only currently supported variant is `bip341` which allows passing a Merkle tree root hash, which is required to implement Taproot signatures as defined in [BIP341](https://github.com/bitcoin/bips/blob/master/bip-0341.mediawiki). The `bip341` variant is anly allowed for `bip340secp256k1` signatures, and if provided the `merkle_root_hash` blob must be exactly 32 bytes long, and should be generated in accordance with BIP341's specification for `taproot_output_script`. If no auxiliary parameter is provided, then `bip340secp256k1` signatures are generated in accordance with BIP340.
+
 Cycles to pay for the call must be explicitly transferred with the call, i.e., they are not automatically deducted from the caller's balance implicitly (e.g., as for inter-canister calls).
 
 ### IC method `http_request` {#ic-http_request}