fix: keep nano-precise timestamps in Java Event APIs

[yaauie]

Release notes

Improves the Java Event API's Event#getEventTimestamp() and Event#setEventTimestamp(Instant) to retain the available precision.

What does this PR do?

• uses Instant directly to keep its nano-precision instead of routing through Instant#toEpochMilli which limits us to milli-precision
• adds tests for observed behaviour
• improves the Event constructor's ability to deal with modern java.time inputs in the @timestamp field (mirroring what was previously done with joda time)

Why is it important/What is the impact to the user?

• When plugins use the Java Event API, Event#getEventTimestamp should be just-as-precise as otherwise available using Event#getField:

  Optional.ofNullable(event.getField("@timestamp"))
      .map(Timestamp#toInstant)
      .orElse(null)

• When plugins use the Java Event API, Event#setEventTimestamp(Instant) should retain the full precision of the provided Instant, as can also be done with Event#setField(Timestamp):

  event.setField("@timestamp", new Timestamp(instant));

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files (and/or docker env variables)
• I have added tests that prove my fix is effective or that my feature works

Related issues

• Add support for Nanosecond-precision timetamps #12797

[mergify]

This pull request does not have a backport label. Could you fix it @yaauie? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• backport-8.x is the label to automatically backport to the 8.x branch.

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label.