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/Makefile.test.include

@@ -146,6 +146,7 @@ BITCOIN_TESTS =\
   test/sigopcount_tests.cpp \
   test/skiplist_tests.cpp \
   test/sock_tests.cpp \
+  test/span_tests.cpp \
   test/streams_tests.cpp \
   test/sync_tests.cpp \
   test/system_tests.cpp \