splice: Allow splice_update to return signatures

This is needed to all multi-channel splices. When channeld can return the signatures to the user (based on signing order precedent), it now does from splice_update.

Additionally, we move sending of the initial psbt from splice_init down to splice_update. This is also necessary for correct psbt diff detection during multi-channel splices.

Changelog-Changed: splice_update can in some cases now return the remotely partiall signed psbt to the user, if so `signtures_secured` will be true.

.msggen.json

@@ -3147,7 +3147,8 @@
         },
         "Splice_updateResponse": {
             "Splice_Update.commitments_secured": 2,
-            "Splice_Update.psbt": 1
+            "Splice_Update.psbt": 1,
+            "Splice_Update.signatures_secured": 3
         },
         "StaticbackupResponse": {
             "StaticBackup.scb[]": 1
@@ -10929,6 +10930,10 @@
             "added": "pre-v0.10.1",
             "deprecated": null
         },
+        "Splice_Update.signatures_secured": {
+            "added": "v24.11",
+            "deprecated": null
+        },
         "StaticBackup": {
             "added": "pre-v0.10.1",
             "deprecated": null