feat: Support outputting stringData from secretGenerator

[stealthybox]

This allows users to pass stringData: true to the secretGenerator, alongside the type option.
When enabled, UTF-8 strings are output in plainText stringData, and non-UTF strings and any binary data fallback to the default behavior of being base64 encoded in data.

This is very similar to the default, kyaml behavior of loading values into ConfigMap's data and binaryData fields.

This feature provides a general U/X improvement for people using kustomize interactively, and also allows Flux users to template into generated secrets.
(Flux users already can and do template into ConfigMaps, so we want to give people more secure mechanisms in kustomize-controller)

• test: add secretGenerator testcases
• feat: support stringData in secretGenerator
• test: test stringData in secretGenerator

resolves #5142 #1444 #1261

[k8s-ci-robot]

This PR has multiple commits, and the default merge method is: merge.
You can request commits to be squashed using the label: tide/merge-method-squash

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[koba1t]

/assign

[matheuscscp]

LGTM 🚀

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: matheuscscp, stealthybox
Once this PR has been reviewed and has the lgtm label, please ask for approval from koba1t. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment