enhance(workers): flags for new filters

[ecrupper]

Examples:

vela get workers  # returns all workers

vela get workers --active true  # returns active workers

vela get workers --active true --since 15m  # returns active workers that have checked in since 15 minutes ago

vela get workers --since 2020-03-20T12:00:00 # returns all workers that have checked in since the pandemic

I didn't find any sort of "fallback" option in urfave/cli to make the parseUnix bit any prettier, but I thought that would be convenient. Very similar to docker logs <container> --since <duration OR timestamp>

[codecov]

Codecov Report

Attention: 14 lines in your changes are missing coverage. Please review.

Comparison is base (1a00c3b) 85.78% compared to head (4b5c051) 85.68%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #517      +/-   ##
==========================================
- Coverage   85.78%   85.68%   -0.11%     
==========================================
  Files         147      147              
  Lines        6425     6476      +51     
==========================================
+ Hits         5512     5549      +37     
- Misses        754      764      +10     
- Partials      159      163       +4     

Files	Coverage Δ
action/worker/get.go	93.47% <100.00%> (+0.97%)	⬆️
action/worker/update.go	100.00% <100.00%> (ø)
command/worker/update.go	75.75% <37.50%> (-12.71%)	⬇️
command/worker/get.go	75.40% <77.50%> (+1.49%)	⬆️

[plyr4]

sweet