Add flag to opt out of existence check when switching namespaces

[tomasaschan]

We have a tool that chooses a cluster and namespace based on runtime data, and
by the time we have decided what cluster and namespace to connect to, we already
know that the namespace exists in the cluster.

We want to extend this tool to support multiple switching mechanisms (rather than
just kubectl config use-context and kubectl config set-context), and but the
existence check for the namespace is prohibitively slow on some of our clusters.
To avoid paying that penalty, this change adds a flag to switch ns which allows
the user to opt out of the existence check.

λ switch ns does-not-exist
namespace "does-not-exist" not foundexit status 1

λ switch ns does-not-exist --check-existence=false

[danielfoehrKn]

Thank you very much for the PR. Could you try to rebase it on master - golangci-lint is failing for this build. Master contains an updated version of golangci-lint. Thank you!

[tomasaschan]

@danielfoehrKn Thanks for taking a look! Rebased now.

This seems related to #147 in that it solves (or, rather, works around) the same issue, namely namespace switching being slow on some clusters. We also have many CRDs in some of our clusters, so it's possible #147 would be enough for our immediate needs, but we also have some clusters with many namespaces (~thousands) so I worry that even if API discovery is made fast enough, the pure listing of those namespaces will still be noticeable. So I'd love to see both of these PRs merged - then I can experiment and see what seems to work best.