[backport#16798] Refactor rawtransaction_util's SignTransaction to separate prevtx parsing
Summary:
Refactor rawtransaction_util's SignTransaction to have previous tx parsing be separate (Andrew Chow)
Pull request description:
Currently the `SignTransaction` function has to handle both the actual signing and parsing of previous transaction data. This PR splits it so that `SignTransaction` only handles the signing itself and adds a `ParsePrevouts` function which handles parsing the prevtx information. This allows for `SignTransaction` to just take any `SigningProvider`. Split from #16341
Backport of Core PR16798
Depends on D7114
Test Plan:
ninja check check-functional
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D7118