rpc: various fixups for dumptxoutset

Summary:

• Actually generate an assumeutxo hash and display it
• Add nchaintx to output (necessary for use in chainparams entry)

The source PR actually duplicated the "path" field in the output. Don't do that.

This is a backport of core#23155

Depends on D14657

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14669