Add a gettime RPC

Summary:
There is currently ne easy way to get the adjusted time of the node, so let's add an RPC to gather the data.

Depends on D14151

Test Plan:

./test/functional/test_runner.py rpc_misc

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14152