wallet: accurately account for the size of the witness stack

When estimating the maximum size of an input, we were assuming the
number of elements on the witness stack could be encode in a single
byte. This is a valid approximation for all the descriptors we support
(including P2WSH Miniscript ones), but may not hold anymore once we
support Miniscript within Taproot descriptors (since the max standard
witness stack size of 100 gets lifted).

It's a low-hanging fruit to account for it correctly, so just do it now.

src/wallet/test/walletload_tests.cpp

@@ -33,6 +33,7 @@ public:
     void ExpandPrivate(int pos, const SigningProvider& provider, FlatSigningProvider& out) const override {}
     std::optional<int64_t> ScriptSize() const override { return {}; }
     std::optional<int64_t> MaxSatisfactionWeight(bool) const override { return {}; }
+    std::optional<int64_t> MaxSatisfactionElems() const override { return {}; }
 };
 
 BOOST_FIXTURE_TEST_CASE(wallet_load_descriptors, TestingSetup)