multi: introduce and use new TapTweak and SignMethod fields

In this commit, we add a new field `TapTweak` to be used for key path
spends. Before this commit, we'd overload the existing `WitnessScript`
field to pass this information to the signing context. This was
confusing as for tapscript spends, this was the leaf script, which
mirrors the other script based spending types.

With this new filed, users need to set this to the script root for
keypath spends where the output key commits to a real merkle root, and
nothing when bip 86 spending is being used.

To make the signing even more explicit, we also add a new field called
sign_method with an enum type that differentiates between the different
segwit v0 and v1 signing methods.

Fixes https://github.com/lightningnetwork/lnd/issues/6446.

lnrpc/signrpc/signer.swagger.json

@@ -731,10 +731,15 @@
           "format": "byte",
           "description": "A private key that will be used in combination with its corresponding\nprivate key to derive the private key that is to be used to sign the target\ninput. Within the Lightning protocol, this value is typically the\ncommitment secret from a previously revoked commitment transaction. This\nvalue is in combination with two hash values, and the original private key\nto derive the private key to be used when signing.\n\nk = (privKey*sha256(pubKey || tweakPub) +\ntweakPriv*sha256(tweakPub || pubKey)) mod N"
         },
+        "tap_tweak": {
+          "type": "string",
+          "format": "byte",
+          "description": "The 32 byte input to the taproot tweak derivation that is used to derive\nthe output key from an internal key: outputKey = internalKey +\ntagged_hash(\"tapTweak\", internalKey || tapTweak).\n\nWhen doing a BIP 86 spend, this field can be an empty byte slice.\n\nWhen doing a normal key path spend, with the output key committing to an\nactual script root, then this field should be: the tapscript root hash."
+        },
         "witness_script": {
           "type": "string",
           "format": "byte",
-          "description": "The full script required to properly redeem the output. This field will\nonly be populated if a p2tr, p2wsh or a p2sh output is being signed. In case\ntaproot_key_spend is set to true then this value must correspond to the\ntaproot root hash (in case of a tapscript output) or the tap hashed internal\npublic key (in case of a BIP-0086 output)."
+          "description": "The full script required to properly redeem the output. This field will\nonly be populated if a p2tr, p2wsh or a p2sh output is being signed. If a\ntaproot script path spend is being attempted, then this should be the raw\nleaf script."
         },
         "output": {
           "$ref": "#/definitions/signrpcTxOut",
@@ -750,9 +755,9 @@
           "format": "int32",
           "description": "The target input within the transaction that should be signed."
         },
-        "taproot_key_spend": {
-          "type": "boolean",
-          "description": "Indicates that this should produce a signature that can be used for the key\nspend path of a taproot input. This requires the witness_script field to be\nset to the taproot root hash (in case of a tapscript output) or the tap\nhashed internal public key (in case of a BIP-0086 output)."
+        "sign_method": {
+          "$ref": "#/definitions/signrpcSignMethod",
+          "description": "The sign method specifies how the input should be signed. Depending on the\nmethod, either the tap_tweak, witness_script or both need to be specified.\nDefaults to SegWit v0 signing to be backward compatible with older RPC\nclients."
         }
       }
     },
@@ -788,6 +793,17 @@
         }
       }
     },
+    "signrpcSignMethod": {
+      "type": "string",
+      "enum": [
+        "SIGN_METHOD_WITNESS_V0",
+        "SIGN_METHOD_TAPROOT_KEY_SPEND_BIP0086",
+        "SIGN_METHOD_TAPROOT_KEY_SPEND",
+        "SIGN_METHOD_TAPROOT_SCRIPT_SPEND"
+      ],
+      "default": "SIGN_METHOD_WITNESS_V0",
+      "description": " - SIGN_METHOD_WITNESS_V0: Specifies that a SegWit v0 (p2wkh, np2wkh, p2wsh) input script should be\nsigned.\n - SIGN_METHOD_TAPROOT_KEY_SPEND_BIP0086: Specifies that a SegWit v1 (p2tr) input should be signed by using the\nBIP0086 method (commit to internal key only).\n - SIGN_METHOD_TAPROOT_KEY_SPEND: Specifies that a SegWit v1 (p2tr) input should be signed by using a given\ntaproot hash to commit to in addition to the internal key.\n - SIGN_METHOD_TAPROOT_SCRIPT_SPEND: Specifies that a SegWit v1 (p2tr) input should be spent using the script\npath and that a specific leaf script should be signed for."
+    },
     "signrpcSignReq": {
       "type": "object",
       "properties": {