fix: allow higher resolution timestamps

[jm-clius]

This fix allows higher precision Timestamps

Wakurtosis reported that all timestamps logged by nwaku were in second resolution (no fractional seconds), which is too imprecise for testing latency, identifying messages etc.

It turns out that our Timestamp construction converted the more precise floatseconds (with fraction) to int64 before multiplying, resulting in the fractional part always being truncated

I've also:

• removed the unused util functions for micro- and milliseconds. I believe a nwaku Timestamp should always be nanoseconds.
• made the generic generation function more strict in requiring SomeNumber types in input.
• added basic test suite

[LNSD]

LGTM ✅

[jm-clius]

Whoops. Naively assumed after local testing that CI will pass. Will investigate the failing tests.

[alrevuelta]

apart from the ci issue, lgtm!

[jm-clius]

Ci fixed, with higher resolution one test failed because it assumed several instructions can be completed in 1 nanosecond. :)