Support defining a KubeConfig Secret data key

[nstogner]

Fixes #458.

[nstogner]

I didn't see a test suite where I could add coverage for this new field. Let me know if I missed something.

[nstogner]

Can I get a review on this PR?

PS: make test is failing. Not sure why at the moment:

make test
cd api; rm -f go.sum; go mod tidy -compat=1.17
rm -f go.sum; go mod tidy -compat=1.17
cd api; /Users/nick/go/bin/controller-gen object:headerFile="../hack/boilerplate.go.txt" paths="./..."
go fmt ./...
cd api; go fmt ./...
go vet ./...
cd api; go vet ./...
/Users/nick/go/bin/controller-gen crd:crdVersions=v1 rbac:roleName=manager-role paths="./..." output:crd:artifacts:config="config/crd/bases"
cd api; /Users/nick/go/bin/controller-gen crd:crdVersions=v1 rbac:roleName=manager-role paths="./..." output:crd:artifacts:config="../config/crd/bases"
/Users/nick/go/bin/gen-crd-api-reference-docs -api-dir=./api/v2beta1 -config=./hack/api-docs/config.json -template-dir=./hack/api-docs/template -out-file=./docs/api/helmrelease.md
I0423 16:20:54.985522   43765 main.go:129] parsing go packages in directory ./api/v2beta1
I0423 16:20:55.089645   43765 main.go:231] using package=./api/v2beta1
F0423 16:20:55.097017   43765 main.go:503] type invalid type has kind=Unsupported which is unhandled
make: *** [api-docs] Error 255