[Housekeeping] Add precheck in demo and sandox commands for local kubeconfig

[pmahindrakar-oss]

Describe the issue

sandbox and demo clusters copies the kubecontext to connect to the cluster into the local machines default kubecontext and switches over to it.

In case the kubecontext doesn't exist locally then the command fails at the this step which is after sandbox is successfully up

Add this precheck for demo and sandbox both
https://github.com/flyteorg/flytectl/blob/master/cmd/demo/start.go#L79
https://github.com/flyteorg/flytectl/blob/master/cmd/sandbox/start.go#L83

What if we do not do this?

User experience is bad as it will always fail for him

Related component(s)

No response

Are you sure this issue hasn't been raised already?

• Yes

Have you read the Code of Conduct?

• Yes

[Icelain]

By this do you mean to check if the Config struct here is empty/non optional fields are filled? - https://github.com/flyteorg/flytectl/blob/master/cmd/config/subcommand/sandbox/sandbox_config.go#L6

[pmahindrakar-oss]

@Icelain I meant we don't do a check for local kubecontext and use the k8sclient to copy over the sandbox context to local kube context https://github.com/flyteorg/flytectl/blob/d2baa0907f7b4ca527bbcc5677483ae8692fd7e6/pkg/sandbox/start.go#L265
and which is after all the sandbox cluster is up and running and fails at these last few steps.

Instead of failing here, it should fail early by validating the existence of the local kube context and if none exist then fail the sandbox/demo cluster creation as these are prerequisite for this command

The task here is to add this validation

[daniel-shuy]

I've created a PR for this at flyteorg/flytectl#363

@pmahindrakar-oss can you help to review it?