Save both initial and final bold reference files

[effigies]

What would you like to see added in fMRIPrep?

We currently save a final boldref, where SDC has been applied. This is useful for BOLD-T1w coregistration, but not for HMC. The initial boldref should be used in that case.

We should save both, so one or both may be provided as precomputed derivatives.

This will be necessary in #2913, but can be done separately, if there's call for another release before that's done.

Do you have any interest in helping implement the feature?

Yes

Additional information / screenshots

No response

[effigies]

Done in #2913.