victoria-metrics-single: add config support for relabelConfig

[belm0]

the chart values should support a global relabel config for ingested metrics, similar to the scrape variable (generating a configMap, etc.)

https://docs.victoriametrics.com/#relabeling

currently we're using --relabelConfig and a separately deployed file, but I'd like a coherent deployment including a configMap for relabel config

(Note that the victoria-metrics-agent chart supports relabel configmap, but victoria-metrics-single does not.)