add: added service account to cloud build config

[OladapoAjala]

Fixes: #7787
Related: Relevant tracking issues, for context
Merge before/after: Dependent or prerequisite PRs

Description

This PR adds a Service Account field to Google Cloud Build configurations.
It allow users set which service account (different from the default) is used by
cloud build.

User facing changes (remove if N/A)

Follow-up Work (remove if N/A)

[codecov]

Codecov Report

Merging #7843 (a11bfaa) into main (290280e) will decrease coverage by 3.83%.
The diff coverage is 54.09%.

@@            Coverage Diff             @@
##             main    #7843      +/-   ##
==========================================
- Coverage   70.48%   66.64%   -3.84%     
==========================================
  Files         515      593      +78     
  Lines       23150    28716    +5566     
==========================================
+ Hits        16317    19138    +2821     
- Misses       5776     8172    +2396     
- Partials     1057     1406     +349     

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/credits/export.go	0.00% <0.00%> (ø)
cmd/skaffold/app/cmd/deploy.go	40.90% <0.00%> (-12.94%)	⬇️
cmd/skaffold/app/cmd/test.go	44.44% <0.00%> (ø)
cmd/skaffold/skaffold.go	0.00% <0.00%> (ø)
cmd/skaffold/app/cmd/inspect_tests.go	62.50% <14.28%> (-1.14%)	⬇️
cmd/skaffold/app/cmd/render.go	35.48% <18.18%> (-5.90%)	⬇️
cmd/skaffold/app/cmd/lsp.go	28.12% <28.12%> (ø)
cmd/skaffold/app/cmd/run.go	64.28% <33.33%> (-9.63%)	⬇️
cmd/skaffold/app/cmd/fix.go	56.41% <37.50%> (-20.07%)	⬇️
cmd/skaffold/app/cmd/verify.go	41.17% <41.17%> (ø)
... and 367 more

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[aaron-prindle]

Thanks for the PR @OladapoAjala! Currently while this PR adds serviceAccount to the schema, it does not plumb the value through our GoogleCloudBuild inputs to the gcloud sdk (which calls the CloudBuild API). If you could also plumb this value through the buildSpec here:
https://github.com/GoogleContainerTools/skaffold/blob/main/pkg/skaffold/build/gcb/spec.go#L39

Something like plumbin the serviceAccount through the necessary gcb code (buildSpec function, etc.) and adding
buildSpec.ServiceAccount = serviceAccount
to the end of the function buildSpecthen the serviceAccount will actually get propogated to the CloudBuild call

[OladapoAjala]

Thanks @aaron-prindle I just pushed an updated code.

[aaron-prindle]

LGTM, seeing one test error that I believe is a flake (not related to the changes you have made). I am restarting the test that failed and hope it passes. Once all the tests are green we can merge