fix: check platform values for known OS and Arch, and fail fast

[gsquared94]

Fixes: #7813

Description

Check the values provided to --platform flag and fail fast if unknown.

Testing instruction:

• In the integration/examples/getting-started project run:

  skaffold build --platform=foo/arm64
  

• You should get the following error:

  creating runner: getting target platforms: failed to parse platforms: unable to recognise platform "foo/arm64": unknown operating system "foo". Check that the value provided to --platform flag is a valid platform and formatted correctly, like linux/amd64, linux/arm64, linux/arm/v7, etc..
  

• Again in the integration/examples/getting-started project run:

  skaffold build --platform=linux/foo
  

• You should get the following error:

  creating runner: getting target platforms: failed to parse platforms: unable to recognise platform "linux/foo": unknown architecture "foo". Check that the value provided to --platform flag is a valid platform and formatted correctly, like linux/amd64, linux/arm64, linux/arm/v7, etc..
  

[codecov]

Codecov Report

Merging #7817 (c2ac3b4) into main (290280e) will decrease coverage by 3.86%.
The diff coverage is 53.59%.

@@            Coverage Diff             @@
##             main    #7817      +/-   ##
==========================================
- Coverage   70.48%   66.62%   -3.87%     
==========================================
  Files         515      593      +78     
  Lines       23150    28668    +5518     
==========================================
+ Hits        16317    19099    +2782     
- Misses       5776     8164    +2388     
- Partials     1057     1405     +348     

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 365 more

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