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
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Backport of Core PR16798
Depends on D7114