Add envsubst package

[stefanprodan]

This PR contains a fork of https://github.com/gomodules/envsubst and incorporates drone/envsubst#34.

This permanent fork is needed because envsubst powers a GA feature of Flux, and the Drone's project looks like is no longer maintained.

xref: fluxcd/flux2#4694

[makkes]

I think we should accompany this PR with an upstream PR to gomodules/envsubst to add the feature of drone/envsubst#34 so that we can remove this fork from our repo once gomodules/envsubst has been released with the feature.

[stefanprodan]

@makkes we can't do that, gomodules/envsubst has diverged from upstream and requires all vars to be specified. Also we may want to add features specific to Flux, for example functions to cope with YAML numbers & booleans.

[makkes]

@makkes we can't do that, gomodules/envsubst has diverged from upstream and requires all vars to be specified. Also we may want to add features specific to Flux, for example functions to cope with YAML numbers & booleans.

Ah, I understand. So this is meant to be a permanent fork. 👍🏻

[souleb]

LGTM!