hcl2template: add strcontains function

[mogrogan]

The strcontains function check if a sub string is a indeed a subset of a given string.

closes #13191

This function was tested inside the packer console command:

$ ./bin/packer console -config-type=hcl2
> strcontains("abcd", "ab")
> true
> strcontains("abcd", "jk")
false

[hashicorp-cla-app]

All committers have signed the CLA.

[lbajolet-hashicorp]

Hey @mogrogan,

Thanks for the PR, I left a nitpick on how the error checking steps are written in the related test, nothing major, feel free to discuss or implement that, let me know how you want to address that.

Besides that, LGTM, so I'm pre-approving to not be a blocker later on.

[lbajolet-hashicorp]

Thanks for the reroll @mogrogan, this LGTM, merging now!

[github-actions]

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