Add PIV support for Kube Access with tsh proxy kube and Teleport Connect

[Joerger]

Use a plain RSA key for the local kube proxy to fix Hardware Key support for tsh proxy kube and Teleport Connect.

[greedy52]

Could you also update it here:

teleport/lib/teleterm/gateway/kube.go

Lines 60 to 65 in d3c321c

	// A key is required here for generating local CAs. It can be any key.
	// Reading the provided key path to avoid generating a new one.
	key, err := keys.LoadPrivateKey(k.cfg.KeyPath)
	if err != nil {
	return nil, trace.Wrap(err)
	}

Thanks!

[Joerger]

@greedy52 Nice catch, thanks!

[r0mant]

@Joerger Will it work in Connect too?

[Joerger]

@Joerger Will it work in Connect too?

Yes

[public-teleport-github-review-bot]

@Joerger See the table below for backport results.

Branch	Result
branch/v13	Failed