WIP: Better Kubeconfig support

[RothAndrew]

Description

Update the getRestConfig() function to provide Kubeconfig support that is closer to the capabilities of kubectl. Specifically, the ability to have multiple files specified in the KUBECONFIG environment variable (which is a default given the way it happens in client-go

Related Issue

Fixes #373

Type of change

• New feature (non-breaking change which adds functionality)

Checklist before merging

• Tests have been added/updated as necessary (add the needs-tests label)
• Documentation has been updated as necessary (add the needs-docs label)
• (Optional) Changes have been linted locally with golangci-lint. (NOTE: We haven't turned on lint checks in the pipeline yet so linting may be hard if it shows a lot of lint errors in places that weren't touched by changes. Thus, linting is optional right now.)

[RothAndrew]

I don't think this needs an ADR, so I'll go ahead and pull that label.

[RothAndrew]

Updated docs. Removing the needs-docs label.

Looking at what changes are needed for tests, if any.

[RothAndrew]

Superseded by #407. Will do a new PR to add an E2E test for nonstandard KUBECONFIG files after #407 merges