Add templatestring function

[nitrocode]

This allows us to fully deprecate the template provider by allowing us to templatize a string.

✗ go install .
✗ ~/go/bin/terraform console
> templatestring("Hello, $${name}!", { name = "Jodie" })
"Hello, Jodie!"

This would be great if we could get this function could be backported to previous versions of terraform to ensure full deprecation of the template provider.

Edit:

• Closes Render templates in a different module to where they are defined #26838
• I also found this older PR lang/funcs: add template function #24978 that creates the template function.

[hashicorp-cla]

All committers have signed the CLA.

[nitrocode]

We decided to go with a forked template provider with an m1 release instead.

https://gist.github.com/nitrocode/cf40a24054a66afe2b19ca54b7be5d68

[github-actions]

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