[jib-cli] Add option to create an output JSON file that contains image metadata for the built image

[mpeddada1]

For #3183

Usage:
Calling jib build --target=${IMAGE_REFERENCE} --image-metadata-out=/my/directory/jib-image.json will result in the creation of jib-image.json (with image details such as image digest, id, tags) in my/directory/, if the directory exists.
Same for jib jar.

[codecov]

Codecov Report

Merging #3187 (aaea7c0) into master (c620122) will decrease coverage by 0.00%.
The diff coverage is 50.00%.

@@             Coverage Diff              @@
##             master    #3187      +/-   ##
============================================
- Coverage     71.24%   71.24%   -0.01%     
- Complexity     2326     2328       +2     
============================================
  Files           279      279              
  Lines          9836     9844       +8     
  Branches        977      977              
============================================
+ Hits           7008     7013       +5     
- Misses         2491     2493       +2     
- Partials        337      338       +1     

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/com/google/cloud/tools/jib/cli/Build.java	18.91% <0.00%> (-0.53%)	4.00 <0.00> (ø)
.../main/java/com/google/cloud/tools/jib/cli/Jar.java	43.75% <0.00%> (-0.70%)	14.00 <0.00> (ø)
...in/java/com/google/cloud/tools/jib/cli/JibCli.java	73.07% <80.00%> (+1.64%)	4.00 <1.00> (+1.00)
...m/google/cloud/tools/jib/cli/CommonCliOptions.java	96.61% <100.00%> (+0.05%)	41.00 <1.00> (+1.00)

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[chanseokoh]

Let's update CHANGELOG.