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. If karydia is configured as mutating and validating webhook, karydia acts accordingly and patches the object.

Checklist

Before submitting this PR, please make sure:

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