MQ-1267 Change queues oldestMessageTimestamp to use kj::Date

[KennethRuan]

Summary

In previous PRs, new metrics were added to the Queues API surface.

#6246
#6354
#6339

As pointed out in #6246, it would be clearer to return a JS Date for oldestMessageTimestamp. This PR implements that change so that oldestMessageTimestamp in the three APIs modified above, all return Date | undefined instead of number.

Changes

This PR includes changes to queue.c++ and queue.h features that are gated behind an experimental flag, as such they are non-breaking. We also make a correction to the metrics() method to use JSG_REQUIRE_NONNULL (see here).

Sentinel Value
We wanted to surface oldestMessageTimestamp as Date | undefined to the user. However, since Date(0) (JS Date at the UNIX epoch) and undefined appear the same over the wire, we opt to treat the zero value as a sentinel and manually set it to kj::none so that it appears as undefined to the end user.

An additional test was also added to ensure that the sentinel value is handled properly.

Testing

• bazel test //src/workerd/api/tests:queue-metrics-sentinel-test@
• bazel test //src/workerd/api/tests:queue-metrics-sentinel-test@all-compat-flags