Add Cmd validation for v1alpha2 Template commands

[chrisdoherty4]

We expect action commands configured on templates to be unix paths that reference an executable. This adds a validation to ensure its a single path.

The command will translate in implementation to a Docker entrypoint which has 2 forms. We're only allowing a single argument as the entrypoint that can accept a number of args <cmd> <arg1> ...<argN> for simplicity.

[codecov]

Codecov Report

Merging #695 (538ab67) into main (d8f27a7) will not change coverage.
The diff coverage is n/a.

❗ Current head 538ab67 differs from pull request most recent head 03aafc9. Consider uploading reports for the commit 03aafc9 to get more accurate results

@@           Coverage Diff           @@
##             main     #695   +/-   ##
=======================================
  Coverage   49.30%   49.30%           
=======================================
  Files          18       18           
  Lines         860      860           
=======================================
  Hits          424      424           
  Misses        428      428           
  Partials        8        8           

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more