routerrpc: extend HTLC forward interceptor resp with modification fields

This commit extends the forward HTLC intercept response with fields that
can be used in conjunction with a `ResumeModified` action to modify the
intercepted HTLC p2p message.

lnrpc/routerrpc/router.proto

@@ -988,6 +988,8 @@ message ForwardHtlcInterceptRequest {
 ForwardHtlcInterceptResponse enables the caller to resolve a previously hold
 forward. The caller can choose either to:
 - `Resume`: Execute the default behavior (usually forward).
+- `ResumeModified`: Execute the default behavior (usually forward) with HTLC
+                    field modifications.
 - `Reject`: Fail the htlc backwards.
 - `Settle`: Settle this htlc with a given preimage.
 */
@@ -1018,12 +1020,36 @@ message ForwardHtlcInterceptResponse {
     // For backwards-compatibility reasons, TEMPORARY_CHANNEL_FAILURE is the
     // default value for this field.
     lnrpc.Failure.FailureCode failure_code = 5;
+
+    // The amount that was set on the p2p wire message of the incoming HTLC.
+    // This field is ignored if the action is not RESUME_MODIFIED or the amount
+    // is zero.
+    uint64 in_amount_msat = 6;
+
+    // The amount to set on the p2p wire message of the resumed HTLC. This field
+    // is ignored if the action is not RESUME_MODIFIED or the amount is zero.
+    uint64 out_amount_msat = 7;
+
+    // Any custom records that should be set on the p2p wire message message of
+    // the resumed HTLC. This field is ignored if the action is not
+    // RESUME_MODIFIED.
+    map<uint64, bytes> out_wire_custom_records = 8;
 }
 
 enum ResolveHoldForwardAction {
+    // SETTLE is an action that is used to settle an HTLC instead of forwarding
+    // it.
     SETTLE = 0;
+
+    // FAIL is an action that is used to fail an HTLC backwards.
     FAIL = 1;
+
+    // RESUME is an action that is used to resume a forward HTLC.
     RESUME = 2;
+
+    // RESUME_MODIFIED is an action that is used to resume a hold forward HTLC
+    // with modifications specified during interception.
+    RESUME_MODIFIED = 3;
 }
 
 message UpdateChanStatusRequest {