Optionally allow ImportScripts to set script creation timestamp

Summary:
Behavior changes:

• scripts imported in importmulti that are not explicilty scriptPubKeys will have timestamps set for them

This is a partial backport of Core PR16301 : https://github.com/bitcoin/bitcoin/pull/16301/commits/78941da5baf6244c7c54e86cf8ce3e09ce60c239

Depends on D6414

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6415