[backport#16377 2/2][rpc] fundrawtransaction: add_inputs option to control automatic input adding

Summary:

Backport of Core PR16377

Depends on D8659

Note: replaced a couple unused i for-loop variables in functional tests that the linter complained about with _'s

Test Plan:

ninja all check check-functional

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8660