Simplify the benchmark with the following changes:
- Set the deterministic private key using uint256::ONE,
put it in a `FlatSigningProvider` instance for easier signing
- Use `GetScriptForDestination` for creating the output script
- Use `SignTransaction` to sign, instead of doing it manually
(also removes the need to caclulate the public key hash manually)
- Pass standard script verification flags instead of combining them manually
These steps, in particular the generalized signing, prepare the
benchmarking extension for a different script type (P2TR key-path) in
the next commit.
dd93362a1d