Add support for a `now` built-in method for warrant policies

[kkajla12]

Is your feature request related to a problem? Please describe.
In some use cases, access rules should only apply to a subject (user) for a specific time period each day/week/month/year. For example, a contract employee might only have access to a particular page in a dashboard on Monday each week. In order to write policies like this, the policy runtime needs to know the time at which the policy is being evaluated.

Describe the solution you'd like
To support writing policies with this type of behavior, we should add support for a built-in now method that returns the current time during evaluation of the policy. Developers can then use this timestamp to implement all sorts of time-based functionality like the example above and more.

Describe alternatives you've considered
Without the ability to get the current timestamp at time of policy evaluation, there is no alternative solution currently.

Additional context
N/A