Add time helpers for std::chrono::steady_clock and FastRandomContext::rand_uniform_delay #24921

A steady clock can be used in the future for the scheduler, for example.

A random uniform delay applied to a time point can be used in the future for time points passed to the scheduler, or delays in net processing.

Currently they are unused outside of tests, but if they turn out unused in the future (unlikely), they can trivially be removed again. I am splitting them out, so that several branches/pulls can build on top of them without duplicating the commits.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #24974 (refactor: Make FEELER_SLEEP_WINDOW type safe (std::chrono) by MarcoFalke)
• #24697 (refactor address relay time by MarcoFalke)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

Concept ACK. I'm lost as to which time PR to look at first at this point, fwiw.

I think we need both, but this one is blocking the addr relay refactor, so review here is minimally more appreciated.

Added doxygen to Now.

ACK fa4fb8d98b7e8e5ea2db35bf239fa7f248da5d8e