This allows the caller to differentiate between an invalid
PSBT/arguments and whether a given struct is present or not.

Rather than forcing the caller to remove these before setting the tx, ignore
them when cloning into the global tx. Note that any global tx in a serialized
PSBT v0 must still not contain these tx input elements as this is
invalid according to BIP-174.

(Strictly speaking, witnesses should never be present in the global tx
because it should be serialized without them. An upcoming change adds a
test case for this).

Expose this by reusing the PSBT parsing code.

Also adds num_items and length getters.

PSBTv2 has a minimum tx version requirement of 2, in order to support
its ability to negotiate locktimes. When upgrading a v0 PSBT to v2,
automatically increase the tx version to comply with the v2 spec.

This means that code which internally upgrades to v2 for processing, then
downgrades to v0 later should either:
- Do nothing, and accept that the tx version will always be updated
- Reject PSBTs containing a tx version less than 2, or
- Save and restore the tx version manually, if they are assured that no
  v2 tx features are used.