Improve usability of func CLI for IDE plugins

[matejvasek]

Changes

• 🎁 Better input handling for non terminal stdio. Useful for scripting with func.
• 🎁 Make not implemented error of cred-helper non fatal error.

feat: better input handling for non terminal stdio. Useful for scripting with `func`.

[codecov]

Codecov Report

Merging #1091 (1add630) into main (04b32d1) will decrease coverage by 0.16%.
The diff coverage is 10.20%.

@@            Coverage Diff             @@
##             main    #1091      +/-   ##
==========================================
- Coverage   45.01%   44.85%   -0.17%     
==========================================
  Files          64       64              
  Lines        8452     8487      +35     
==========================================
+ Hits         3805     3807       +2     
- Misses       4266     4296      +30     
- Partials      381      384       +3     

Impacted Files	Coverage Δ
cmd/deploy.go	51.40% <0.00%> (-3.39%)	⬇️
docker/creds/credentials.go	69.25% <31.25%> (-1.36%)	⬇️
s2i/builder.go	63.07% <0.00%> (-1.16%)	⬇️

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[matejvasek]

@lance @lkingland PTAL

[lkingland]

/lgtm

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lkingland, matejvasek

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