Add support for extra command line flags for use by filters

[gberche-orange]

Thanks for contributing this helm chart !

In order to configure filters, it would be great to have values.yaml support an additional extraArgs map argument such as -oauth2-tokeninfo-url such as

https://github.com/zalando/skipper/blob/063e1c5edfdb80e2f799345b2ce46a8097a67180/docs/tutorials/auth.md?plain=1#L349-L356

skipper -enable-oauth2-grant-flow \
    -oauth2-auth-url=<OAUTH2_AUTHORIZE_ENDPOINT> \
    -oauth2-token-url=<OAUTH2_TOKEN_ENDPOINT> \
    -oauth2-revoke-token-url=<OAUTH2_REVOKE_TOKEN_ENDPOINT> \
    -oauth2-tokeninfo-url=<OAUTH2_TOKENINFO_ENDPOINT> \
    -oauth2-callback-path=/oauth/callback

into

skipper-helm/skipper/templates/daemonset.yaml

Lines 48 to 71 in 4226348

	args:
	- "skipper"
	- "-kubernetes"
	- "-kubernetes-in-cluster"
	- "-address=:{{- .Values.skipper.webEndpoint.port }}"
	- "-proxy-preserve-host"
	- "-serve-host-metrics"
	- "-enable-ratelimits"
	- "-experimental-upgrade"
	- "-metrics-exp-decay-sample"
	- "-kubernetes-https-redirect=true"
	{{- if .Values.skipper.logLevel }}
	- "-application-log-level={{- .Values.skipper.logLevel }}"
	{{- end }}
	- "-lb-healthcheck-interval=3s"
	- "-enable-connection-metrics"
	- "-reverse-source-predicate"
	{{ if .Values.prometheusOperator.create }}
	- "-metrics-flavour=prometheus"
	- "-support-listener=:{{ .Values.skipper.metricsEndpoint.port }}"
	{{ end }}
	{{- if .Values.skipper.ingressClass }}
	- "--kubernetes-ingress-class={{ .Values.skipper.ingressClass }}"
	{{- end }}

[gberche-orange]

Actually, for my specific use-case, the url would be dynamic and then the command line flag would not be needed, see zalando/skipper#3444

Sorry for the noise