feat: Generate secrets from templating

[motoki317]

Hello ksops team!

This is a (non-breaking) feature pull request to add one advanced feature to the ksops plugin.

I have added usage and usecases to README.md, but in short, this feature allows you to use golang text/template to template a secret from variables read via sops.
Please feel free to correct my English in README if anything feels unnatural, because I am not a native English speaker.

My particular usecases include: templating a Gitea app.ini config file.
The app.ini is large, and has quite a few fields and a number of secret fields I would like to mask in a git repository.
(Since .ini is supported by sops, I guess technically I could use unencrypted_regex to filter out fields in .sops.yaml, but that would get the config file too cluttered. I would personally rather use this templating feature implemented in this PR.)

I have uploaded a built docker image of this PR to ghcr.io/motoki317/ksops so I can try it out in my environment.
You can view my refactor commits using this feature from the following links:
motoki317/manifest@533de9f
motoki317/manifest@570a8f3

I should also note that this was partially inspired by external-secret's advanced templating feature.
https://external-secrets.io/latest/guides/templating/

Thank you in advance!

[motoki317]

@devstein Hi! Would you mind taking a look at this?

[devstein]

Hey @motoki317 thanks for the contribution! I'll take a look this weekend.

I generally want to keep KSOPS lean and simply a wrapper around kustomize and SOPS, but think this could makes sense.