Add implementation for resources secret endpoints

[absoludity]

Description of the change

Following on from #3914, this PR adds the implementations for CreateSecret and GetSecretNames.

Benefits

Can now update the dashboard to use the new endpoints rather than hitting the k8s API server.

Possible drawbacks

Applicable issues

• Ref Replace remaining Dashboard calls to K8s API server #3896

Additional information

[castelblanque]

Great, now Kubeapps is not exposing secret contents anymore, and we are moving away from k8s API server calls.
Thanks!

[absoludity]

Great, now Kubeapps is not exposing secret contents anymore,

Not for the dashboard endpoints that will switch to use this API. Note that the GetResourcesForInstalledPackage still returns the secrets for an installed package just like any other resource (and they're used in the dashboard), but we can eventually remove that too so that we only fetch the actual secret value if the user chooses to view one. But that's for another day.

and we are moving away from k8s API server calls. Thanks!

Yay to that :)

[antgamdia]

Great! Another step forward!

[antgamdia]

In order to avoid harcoding these literals, I opted for adding the corev1 k8s API as a dependency and just did:

		TypeMeta: metav1.TypeMeta{
			Kind:       k8scorev1.ResourceSecrets.String(),
			APIVersion: k8scorev1.SchemeGroupVersion.WithResource(k8scorev1.ResourceSecrets.String()).String(),
		},
...

See what you think, but anyway, I'll try to be as consistent as possible to avoid future issues when upgrading k8s versions.
I mean, I'm happy to change the kapp-related code I wrote to use the literals directly if you think otherwise, but, you know, we better keep it uniform.

Note: this comment also applies to the CreateNamespace function, but it was already merged.

[absoludity]

Sounds good, I'll update to that.

[stale]

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