Distinguish between mutating and validating service token pod webhook.

[marcrahnsap]

Description

Option karydia.gardener.cloud/automountServiceAccountToken=remove-default is only applicable for the mutating webhook. If the mutating hook is not registered at the api server, but the option remove-default is set, karydia will return a meaningful error message and will deny the deployment.

Checklist

Before submitting this PR, please make sure:

• [x] your code builds clean with make test
• [x] you have added unit tests