feat: Add indent function to support multi-line content

[aparra]

We want to extract Lua code from kong.yaml file for two reasons:

1. Better syntax highlight in the IDE for .lua files
2. Write unit tests for the functions

So, we would like to have a setup like that:

kong.yaml

---
_format_version: "3.0"
plugins:
  - name: pre-function
    config:
      log: |
        ${{ env "DECK_FILE_LOG_FUNCTION" | indent 8 }}

file_log.lua

function parse_traceid(str)str = string.sub(str,1,8)
  local uint = 0
  for i = 1, #str do
    uint = uint + str:byte(i) * 0x100^(i-1)
  end
  return string.format("%.0f", uint)
end

kong.log.set_serialize_value("trace_id", parse_traceid(ngx.ctx.KONG_SPANS[1].trace_id))
kong.log.set_serialize_value("span_id", parse_traceid(ngx.ctx.KONG_SPANS[1].span_id))

However, without the indent function, it is not possible to convert the yaml to json. It fails due indentation required by yaml. This issue #616 worked around this problem, replacing yaml with json, where you have better control of quotes.

But we would like consistency in the setup; it means using yaml everywhere. Inspired by k8s helm, where function indent is available https://helm.sh/docs/chart_template_guide/function_list/#indent to deal with multi-line block, I added the indent function to deck.

[CLAassistant]

All committers have signed the CLA.

[aboudreault]

@aparra Thanks for this PR. tested quickly and it seems to work well. I will check to merge this one soon