enhance(library): update Duration() return values

[jbrockopp]

Based off feedback from #222 (comment)

This updates the value returned from Duration() for resources in the github.com/go-vela/types/library package.

The following resources have this function:

• build
• service
• step

Previously, we were returning 0s if the started or finished timestamps weren't set for the resource.

Now, If a resource doesn't have a started timestamp, then an indeterminate value of ... is returned.

Otherwise, if a resource doesn't have a finished timestamp, then the time since the started timestamp is returned.

Otherwise, we return the finished timestamp subtracted by the started timestamp.

[codecov]

Codecov Report

Merging #227 (366253d) into master (0787a67) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #227      +/-   ##
==========================================
+ Coverage   97.05%   97.06%   +0.01%     
==========================================
  Files          53       53              
  Lines        5772     5794      +22     
==========================================
+ Hits         5602     5624      +22     
  Misses        125      125              
  Partials       45       45              

Impacted Files	Coverage Δ
library/build.go	100.00% <100.00%> (ø)
library/service.go	87.10% <100.00%> (+0.26%)	⬆️
library/step.go	100.00% <100.00%> (ø)

[ecrupper]

lgtm