feat(interpolation): add fromFile interpolation helper

[LuBashQ]

1. Description

Add interpolation helper that reads a file path and returns the contents of the file if exists, empty string otherwise. The method is intended to be used for example chained to toJSON helper, given a correctly formatted JSON file.

2. Related issues

#7213

3. About tests

Added unit tests with both existing and non-existing paths.

[richardlt]

Hi, I understand your request about Venom but as this interpolation is also used by CDS api we can't safely approve the PR as we don't want this interpolate to read files in that context.
Venom should use its own set of helpers for that purpose, you can submit a PR on Venom repository if needed.

[LuBashQ]

The reason why the PR was opened here is that venom uses cds' interpolate package to resolve helper functions https://github.com/ovh/venom/blob/f3548d1573594480413f728e4a57f0f5130b36e8/types_executor.go#L275, which are filled in here

cds/sdk/interpolate/interpolate.go

Line 166 in fafc78f

t, err := template.New("input").Funcs(InterpolateHelperFuncs).Parse(input)

.

My question now is? Would it be then a good option to treat the return from interpolate.Do() in Venom as a template and then further process it with the venom-specific helper functions? Correct me if I'm wrong, but from what I understand interpolate.Do() in case of unknown helpers, it will just keep them as-is

cds/sdk/interpolate/interpolate.go

Lines 153 to 160 in fafc78f

	if !defaultIsUsed && (len(unknownVariables) > 0 || len(unknownHelpers) > 0) {
	for _, s := range quotedStuff {
	q := strings.Replace(sm[i][1], `"`+s+`"`, `\"`+s+`\"`, -1)
	input = strings.Replace(input, sm[i][1], q, 1)
	sm[i][1] = q
	}
	input = strings.Replace(input, sm[i][1], "{{\""+sm[i][1]+"\"}}", -1)

, so could it then be processed further by venom?