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/script/descriptor.h

@@ -146,6 +146,15 @@ struct Descriptor {
 
     /** @return The OutputType of the scriptPubKey(s) produced by this descriptor. Or nullopt if indeterminate (multiple or none) */
     virtual std::optional<OutputType> GetOutputType() const = 0;
+
+    /** Get the size of the scriptPubKey for this descriptor. */
+    virtual std::optional<int64_t> ScriptSize() const = 0;
+
+    /** Get the maximum size of a satisfaction for this descriptor, in weight units.
+     *
+     * @param use_max_sig Whether to assume ECDSA signatures will have a high-r.
+     */
+    virtual std::optional<int64_t> MaxSatisfactionWeight(bool use_max_sig) const = 0;
 };
 
 /** Parse a `descriptor` string. Included private keys are put in `out`.