[RPC] Add an uptime command that displays the amount of time that bitcoind has been running

Summary: Backport core's PR10400

Test Plan:

make check
./test/functional/test_runner.py uptime

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1039