fix: add inputDigest tagger alias to customTemplate tagger

[mikekamornikov]

Fixes: #7865

Description
It modifies customTemplate tagger to support inputDigest by default:

• Given template is parsed to understand which variables to expect. This way it's not required to initialize all possible taggers in advance. And it seriously simplifies testing.
• Taggers are created one by one giving higher priority to the ones defined in components.

[codecov]

Codecov Report

Merging #7867 (42f8b05) into main (290280e) will decrease coverage by 3.70%.
The diff coverage is 53.94%.

❗ Current head 42f8b05 differs from pull request most recent head 61309f9. Consider uploading reports for the commit 61309f9 to get more accurate results

@@            Coverage Diff             @@
##             main    #7867      +/-   ##
==========================================
- Coverage   70.48%   66.77%   -3.71%     
==========================================
  Files         515      594      +79     
  Lines       23150    28754    +5604     
==========================================
+ Hits        16317    19201    +2884     
- Misses       5776     8141    +2365     
- Partials     1057     1412     +355     

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]

LGTM! Thanks for this! I have created #7933 to track updating the Skaffold documentation to also include the new INPUT_DIGEST field