[backport#15508] Refactor analyzepsbt for use outside RPC code

Summary:
Refactor the analyzepsbt RPC into (1) an AnalyzePSBT function, which returns
its output as a new strongly-typed PSBTAnalysis struct, and (2) a thin wrapper
which converts the struct into a UniValue for RPC use.

https://github.com/bitcoin/bitcoin/pull/15508/commits/ef22fe8c1f331b4f13f21a54d12030b92e83fbe7

Depends on D6077

This is a partial backport of Core PR15508

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, nakihito

Reviewed By: nakihito

Subscribers: nakihito

Differential Revision: https://reviews.bitcoinabc.org/D6079