Define default resource requests for comonent gardener-extension-admission-shoot-dns

[andrerun]

How to categorize this issue?
/area control-plane
/kind enhancement

What would you like to be added:
The gardener-extension-admission-shoot-dns-service component, running in the garden namespace in the garden runtime cluster, currently does not specify resource requests, and initially runs as a "best effort" pod until it is evicted and VPA sets a value for requests. In theory, we could improve initial pod placement and increase the mean time to first eviction by providing appropriate default requests.

Why is this needed:
That would be a minor improvement and this issue is filed for general tracking purposes, not because of any urgent problem. The suggested improvement should result in a minor reduction in the frequency of eviction-driven webhook latency spikes.