Moves common request proxy utilities to proxyutil package with
support for removing X-Forwarded-Host, X-Forwarded-Port,
X-Forwarded-Proto headers, setting X-Forwarded-For header
and cleaning Cookie header.
Using the proxyutil package to prepare and clean request
headers before resource calls.
Closes#21512
Adds support for collecting metrics from backend plugins and
exposing them thru Grafana's Prometheus metrics endpoint.
Enables to check health of backend plugin by using the route
`/api/plugins/<plugin id>/health`.
Uses sdk v0.6.0.
Closes#20984
