builtins: implement time.now_ns

[DFrenkel]

What is the underlying problem you're trying to solve?

Need implementation of time.now_ns to be used in policies

Expected Outcome

• time.now_ns() can be used in policies
• The result of time.now_ns() call is consistent for the duration of a given evaluation
• Relevant subset of time tests pass: https://github.com/open-policy-agent/opa/tree/main/v1/test/cases/testdata/v1/time