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/wallet/interfaces.cpp

@@ -11,6 +11,7 @@
 #include <policy/fees.h>
 #include <primitives/transaction.h>
 #include <rpc/server.h>
+#include <scheduler.h>
 #include <support/allocators/secure.h>
 #include <sync.h>
 #include <uint256.h>
@@ -212,7 +213,7 @@ public:
         }
         return true;
     }
-    std::vector<WalletAddress> getAddresses() const override
+    std::vector<WalletAddress> getAddresses() override
     {
         LOCK(m_wallet->cs_wallet);
         std::vector<WalletAddress> result;
@@ -585,10 +586,15 @@ public:
     }
     bool verify() override { return VerifyWallets(m_context); }
     bool load() override { return LoadWallets(m_context); }
-    void start(CScheduler& scheduler) override { return StartWallets(m_context, scheduler); }
+    void start(CScheduler& scheduler) override
+    {
+        m_context.scheduler = &scheduler;
+        return StartWallets(m_context);
+    }
     void flush() override { return FlushWallets(m_context); }
     void stop() override { return StopWallets(m_context); }
     void setMockTime(int64_t time) override { return SetMockTime(time); }
+    void schedulerMockForward(std::chrono::seconds delta) override { Assert(m_context.scheduler)->MockForward(delta); }
 
     //! WalletLoader methods
     util::Result<std::unique_ptr<Wallet>> createWallet(const std::string& name, const SecureString& passphrase, uint64_t wallet_creation_flags, std::vector<bilingual_str>& warnings) override