refactor: make GetRand a template, remove GetRandInt

Summary:
makes GetRand a template for which any integral type can be used, where the default behavior is to return a random integral up to the max of the integral unless a max is provided.
This simplifies a lot of code from GetRand(std::numeric_limits<uint64_t>::max() -> GetRand<uint64_t>()

This is a backport of core#24925

Depends on D14827

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14828