Add command to custom artifact dependencies

[priyawadhwa]

Allow users to specify a command to execute to determine dependencies
for a custom artifact. Dependencies must be returned as a valid JSON
array.

Now, this is valid:

custom:
  buildCommand: ./build.sh
  dependencies:
    command: echo ["file1","file2","file3"]

[codecov-io]

Codecov Report

Merging #2095 into master will increase coverage by 0.02%.
The diff coverage is 63.63%.

@@            Coverage Diff            @@
##           master   #2095      +/-   ##
=========================================
+ Coverage   56.18%   56.2%   +0.02%     
=========================================
  Files         180     180              
  Lines        7771    7784      +13     
=========================================
+ Hits         4366    4375       +9     
- Misses       2988    2990       +2     
- Partials      417     419       +2

Impacted Files	Coverage Δ
pkg/skaffold/schema/latest/config.go	100% <ø> (ø)	⬆️
pkg/skaffold/schema/validation/validation.go	96.29% <100%> (ø)	⬆️
pkg/skaffold/build/custom/dependencies.go	75% <60%> (-10.72%)	⬇️
pkg/skaffold/runner/labeller.go	100% <0%> (ø)	⬆️

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[container-tools-bot]

Please visit http://35.236.49.15:1313 to view changes to the docs.

[balopat]

LGTM with minor docs comments