Feature: add filebase64 function

[conorevans]

Signed-off-by: Conor Evans coevans@tcd.ie

Closes #11522

As noted in that issue, Terraform implements this function. About a month ago, the MakeFileFunc in Terraform deviated from hashicorp/go-cty-funcs which Nomad is using and which this PR continues to use. If the code change in Terraform is relevant here, perhaps it should be copied over to hashicorp/go-cty-funcs, but I'll leave that in your hands.

I copied the Doc largely from Terraform, removing some of the content that was Terraform-focused. If it needs to be updated, let me know or make the changes yourself as you see fit. Docs are pretty tricky to get right and you maintainers will know much better than me how you'd like to phrase it.

I didn't add a test here, mostly because there is no existing functions_test file with a Test Table I could easily plug in to, and the Terraform test didn't transfer over. Unfortunately there's also no test in hashicorp/go-cty-funcs. If you want to add one, please do so as you see fit.

[hashicorp-cla]

All committers have signed the CLA.

[tgross]

LGTM!

Sorry about the long delay in reviewing this @conorevans. I've rebased this on main and added a changelog entry. It'll get merged once our docs preview renders and we'll get this out in Nomad 1.3.2. Thanks for your patience!

Bench test verifying this works as intended:

job "httpd" {
...
      env {
        DATA_BASE64 = filebase64("world.txt")
      }
...
}

$ echo 'hello' > world.txt
$ nomad job run ./httpd.nomad
$ nomad job inspect httpd | jq '.Job.TaskGroups[0].Tasks[0].Env'
{
  "DATA_BASE64": "aGVsbG8K"
}

[tgross]

Preview link: https://nomad-e0hmk7os0-hashicorp.vercel.app/docs/job-specification/hcl2/functions/file/filebase64

[github-actions]

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