feat: Add node uptime rpc / output in info command

[magik6k]

Related Issues

Proposed Changes

• Add an Uptime jsonrpc method
• Output uptime from node info commands

Additional Info

This in useful to double check that nodes were restarted after making some changes which could require restarting, for example.

Checklist

Before you mark the PR ready for review, please make sure that:

• All commits have a clear commit message.
• The PR title is in the form of of <PR type>: <area>: <change being made>
  • example: fix: mempool: Introduce a cache for valid signatures
  • PR type: fix, feat, INTERFACE BREAKING CHANGE, CONSENSUS BREAKING, build, chore, ci, docs,perf, refactor, revert, style, test
  • area: api, chain, state, vm, data transfer, market, mempool, message, block production, multisig, networking, paychan, proving, sealing, wallet, deps
• This PR has tests for new functionality or change in behaviour
• If new user-facing features are introduced, clear usage guidelines and / or documentation updates should be included in https://lotus.filecoin.io or Discussion Tutorials.
• CI is green

[jennijuju]

Oh what inspired this?

[magik6k]

I wasn't sure if I have actually restarted my node after updating it, and this seemed like a good use of the 7 minutes of time this took to implement.