[backport#17371 1/2] Accumulate result UniValue in SignTransaction

Summary:
SignTransaction will be called multiple times in the future. Pass
it a result UniValue so that it can accumulate the results of multiple
SignTransaction passes.

https://github.com/bitcoin/bitcoin/pull/17371/commits/4b0c718f8f48c678cbe4575e9a9cf9e62a30f0da

Partial backport of Core PR17371

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7686