Keychain to ignore non-existing secrets

[vpnachev]

What would you like to be added:
Sometimes, the pods refer to image pull secrets that simply do not exist, the image tag resolver and signature verifiers can simply ignore those secrets and work only with the available ones.

The code that needs to be changed is

gardener-extension-shoot-lakom-service/pkg/lakom/resolvetag/admission.go

Lines 158 to 160 in 5ccdfc3

	if err := h.reader.Get(ctx, secretKey, secret); err != nil {
	return nil, err
	}

and

gardener-extension-shoot-lakom-service/pkg/lakom/verifysignature/admission.go

Lines 179 to 181 in 5ccdfc3

	if err := h.reader.Get(ctx, secretKey, secret); err != nil {
	return nil, err
	}

Why is this needed:
To not fail when non-existing image pull secrets are set in the pods spec.