Merge bitcoin/bitcoin#28721: multiprocess compatibility updates

3b70f7b615 doc: fix broken doc/design/multiprocess.md links after #24352 (Ryan Ofsky)
6d43aad742 span: Make Span template deduction guides work in SFINAE context (Ryan Ofsky)
8062c3bdb9 util: Add ArgsManager SetConfigFilePath method (Ryan Ofsky)
441d00c60f interfaces: Rename CalculateBumpFees methods to be compatible with capn'proto (Ryan Ofsky)
156f49d682 interfaces: Change getUnspentOutput return type to avoid multiprocess segfault (Ryan Ofsky)
4978754c00 interfaces: Add schedulerMockForward method so mockscheduler RPC can work across processes (Ryan Ofsky)
924327eaf3 interfaces: Fix const virtual method that breaks multiprocess support (Ryan Ofsky)
82a379eca8 streams: Add SpanReader ignore method (Russell Yanofsky)

Pull request description:

  This is a collection of small changes to interfaces and code which were needed as part of multiprocess PR #10102, but have been moved here to make that PR smaller.

  All of these changes are refactoring changes which do not affect behavior of current code

  ---

  This PR is part of the [process separation project](https://github.com/bitcoin/bitcoin/issues/28722).

ACKs for top commit:
  achow101:
    ACK 3b70f7b615
  naumenkogs:
    ACK 3b70f7b615
  maflcko:
    re-ACK 3b70f7b615  🎆

Tree-SHA512: 2368772b887056ad8a9f84c299cfde76ba45943770e3b5353130580900afa9611302195b899ced7b6e303b11f053ff204cae7c28ff4e12c55562fcc81119ba4c

src/qt/transactiondesc.cpp

@@ -360,12 +360,10 @@ QString TransactionDesc::toHTML(interfaces::Node& node, interfaces::Wallet& wall
         {
             COutPoint prevout = txin.prevout;
 
-            Coin prev;
-            if(node.getUnspentOutput(prevout, prev))
-            {
+            if (auto prev{node.getUnspentOutput(prevout)}) {
                 {
                     strHTML += "<li>";
-                    const CTxOut &vout = prev.out;
+                    const CTxOut& vout = prev->out;
                     CTxDestination address;
                     if (ExtractDestination(vout.scriptPubKey, address))
                     {