descriptor: introduce a method to get the satisfaction size

In the wallet code, we are currently estimating the size of a signed
input by doing a dry run of the signing logic. This is unnecessary as
all outputs we are able to sign for can be represented by a descriptor,
and we can derive the size of a satisfaction ("signature") from the
descriptor itself directly.
In addition, this approach does not scale: getting the size of a
satisfaction through a dry run of the signing logic is only possible for
the most basic scripts.

This commit introduces the computation of the size of satisfaction per
descriptor. It's a bit intricate for 2 main reasons:
- We want to conserve the behaviour of the current dry-run logic used by
  the wallet that sometimes assumes ECDSA signatures will be low-r,
  sometimes not (when we don't create them).
- We need to account for the witness discount. A single descriptor may
  sometimes benefit of it, sometimes not (for instance `pk()` if used as
  top-level versus if used inside `wsh()`).

src/wallet/test/walletload_tests.cpp

@@ -31,6 +31,8 @@ public:
     bool Expand(int pos, const SigningProvider& provider, std::vector<CScript>& output_scripts, FlatSigningProvider& out, DescriptorCache* write_cache = nullptr) const override { return false; };
     bool ExpandFromCache(int pos, const DescriptorCache& read_cache, std::vector<CScript>& output_scripts, FlatSigningProvider& out) const override { return false; }
     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 {}; }
 };
 
 BOOST_FIXTURE_TEST_CASE(wallet_load_descriptors, TestingSetup)