[flagd] Add `$flagd.timestamp` to in-process evalutor

[toddbaert]

The flagd in-process evaluator needs to have $flagd.timestamp added, so that users can write time-based JsonLogic rules.

See here for doc, and here for the flagd-standalone implementation.

You'll need to add a standard unix timestamp (seconds since 1970) to the evaluation context, similar to the flagKey, here.

[DBlanchard88]

ill do it

[toddbaert]

Relates to: open-feature/flagd-testbed#74

[toddbaert]

Done with #512

Thanks @DBlanchard88 !