[util] Add method that returns random time in milliseconds

Summary:
This is a backport of Core PR18038 [2/7]
https://github.com/bitcoin/bitcoin/pull/18038/commits/7e93eecce3bc5a1b7bb0284e06f9e2e69454f5ba
Depends on D9006

Test Plan: ninja

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9007