Add duration as a configurable type for localkube

[r2d4]

Fixes #1712

I refactored a bit of this code. Now it works like this:

1. Check the actual type, this will capture anything we have explicitly in the switch block and set it.
2. If we don't have an explicit entry for the type, fall back to looking at the underlying primitive kind. This might not always work (e.g. for types that have a nontrivial parsers, i.e. time.ParseDuration), but it will work for many types.

Previously, we were checking the underlying primitive kind first, which captured time.Duration (which is a type alias for int) and tried to Atoi("5s").

[codecov-io]

Codecov Report

Merging #1715 into master will increase coverage by 0.26%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1715      +/-   ##
==========================================
+ Coverage   35.93%   36.19%   +0.26%     
==========================================
  Files          51       51              
  Lines        3337     3365      +28     
==========================================
+ Hits         1199     1218      +19     
- Misses       1958     1966       +8     
- Partials      180      181       +1

Impacted Files	Coverage Δ
pkg/util/config.go	63.54% <66.66%> (+1.77%)	⬆️

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