Allow using custom kubectl binary

[steeve]

Description

Add a kubectl attribute to k8s_deploy, allowing to specify a custom kubectl binary.

Fix #33

Related Issue

https://github.com/adobe/rules_gitops/issues/33

Motivation and Context

Allows for easily wrapping or sandboxing the kubectl binary.

How Has This Been Tested?

Tested locally on macOS with a k3s cluster running in Docker for Mac.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] I have signed the Adobe Open Source CLA.
• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

[steeve]

I figure I’d get some feedback before going all the way.

[steeve]

Actually you are right that using a toolchain might be cleaner. I’ll take a look.

[kzadorozhny]

There is another contribution #22 that is left in unmerged state because it is waiting for the toolchain implementation. @michaelschiff is ok with using a fork for now.

I'm not objecting to merging this PR , tough.