Add kops toolbox template docs

[kenden]

Document the template format of new command kops toolbox template (#3040)

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[gambol99]

@kenden ... You just reminded me i probably need add the additional documentation related to PR aaf6143 ... which adds snippets, multiple templates / values, directories and sprig templates methods ..

[gambol99]

LGTM

[chrislovecnm]

Question for you

[chrislovecnm]

Is that read a tab?

[kenden]

Fixed

[chrislovecnm]

/ok-to-test
/assign

[chrislovecnm]

@gambol99 we need release notes for SURE. @kenden do you mind doing another iteration on the docs?

[chrislovecnm]

Holy crap I need to look at your git commit messages @gambol99

Toolbox Templating
Extending the current implementation of toolbox template to include multiple files and snippets. Note, i've removed the requirements for defaults as I think people should be forced to specifically pass them.

- fixing the vetting iseues to the method YamlToJson -> YAMLToJSON
- adding a safety check to ensure templates don't reference an unknown value
- extending the unit test to ensure the above works on main and snippets
- include the ability to specify multiple configuration files, useful for common.yaml and prod.yaml etc

Requested Changes - Toolbox Templating

Added the requested changes

- moved the templater into it's own package rather than using base util
- moved to using the sprig library for additional template function
- note: i couldn't find a native way in sprig to do snippets, also the i've overloaded the indent as it appears to do the indent on all lines rather than on the newline, meaning i'd have to shift my first line back by the indent to get it to work, which seems ugly

[chrislovecnm]

/lgtm

@kenden If I can get you to do another PR on @gambol99 features docs please let me know, otherwise, I will shanghi @gambol99 :) It is always better when an ops user does the docs :)

[k8s-github-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chrislovecnm

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[k8s-github-robot]

Automatic merge from submit-queue.

[gambol99]

I'm gonna update the documentation to reflect features added by aaf6143