To prepare for returning multipath descriptors which will be a shorthand
for specifying 2 descriptors, change ParseScript's signature to return a
pair.

To prepare for multipath descriptors, have ParsePubkey return a pair of
PubkeyProviders and have ParseScript be able to handle them.

Multipath specifiers are derivation path indexes of the form `<r;c>`
used for specifying derivation paths for receiving and change
descriptors. Only one multipath specifier is allowed per PubkeyProvider,
and it must only contain two path indexes. This is syntactic sugar which
is parsed into two distinct descriptors. One descriptor will have all of
the `r` paths, the other all of the `c` paths.

ParseKeypath will always return a pair of keypaths. If no multipath
descriptor is provided, the second one will be a std::nullopt. The
callers of this function are updated to deal with this case and return
multiple PubkeyProviders. Their callers have also been updated to handle
pairs of PubkeyProviders.

When given a descriptor which contins a multipath derivation specifier,
a pair of descriptors will be returned.

When given a multipath descriptor, derive both the receiving and change
descriptors. The derived addresses will be returned in an object
consisting of 2 arrays. For compatibility, when given a single path
descriptor, the addresses are provided in a single array as before.

Instead of applying internal-ness to all keys being imported at the same
time, apply it on a per key basis. So each key that is imported will
carry with it whether it is for the change keypool.

Multipath descriptors will be imported as two separate descriptors with
the first one for receiving addreses, and the second for change
addresses. When importing a multipath descriptor, 'internal' cannot be
specified because of the implicit receive and change descriptors.

Multipath descriptors will be imported as two separate descriptors with
the first for receiving addresses and the second for change. This
mirrors importmulti.

Test that both importmulti and importdescriptors behave as expected when
importing a multipath descriptor.