Allow passing of metrics server secret to flagger

[joedborg]

An attempt at fixing #1671. I'm not much of a Go wizard, so it probably requires some cleanup.

This adds the ability to add --metrics-server-secret-ref to give the ability to auth against external Prometheus endpoints.

[joedborg]

@stefanprodan and chance I can get a review on this please?

[aryan9600]

is it not possible to define custom MetricTemplate objects pointing to the prom server with the appropriate credentials? the query can be copy pasted from flagger's codebase. that's hardcoded anyway: https://github.com/fluxcd/flagger/blob/main/pkg/metrics/observers/istio.go#L27

[joedborg]

@aryan9600 that's what I'm doing as a work around but I'd like the built ins to work as described without having to duplicate code from the repo to get it to work.