Split proxy configuration out of resource handlers

[K-Phoen]

I was starting to feel a bit overwhelmed by the amount of code/methods in resource handlers, which only increases for handlers that need to describe how to proxy resources for grr serve.

In this PR, I suggest to split proxy configuration code out of resource handlers and into a dedicated file/struct.

This is purely a refactoring: no bug was fixed or feature changed/introduced.

As a next step, I would like the individual ProxyConfigurator structs to describe this configuration:

grizzly/pkg/grizzly/server.go

Lines 102 to 148 in eca189d

	var mustProxyGET = []string{
	"/public/*",
	"/api/datasources/proxy/*",
	"/api/datasources/*",
	"/api/plugins/*",
	"/avatar/*",
	}
	var mustProxyPOST = []string{
	"/api/datasources/proxy/*",
	"/api/ds/query",
	}
	var blockJSONget = map[string]string{
	"/api/ma/events": "[]",
	"/api/live/publish": "[]",
	"/api/live/list": "[]",
	"/api/user/orgs": "[]",
	"/api/annotations": "[]",
	"/api/search": "[]",
	"/api/usage/*": "[]",
	"/api/frontend/assets": "{}",
	"/api/org/preferences": "{}",
	"/api/access-control/user/actions": `{"dashboards:write": true}`,
	"/api/prometheus/grafana/api/v1/rules": `{
	"status": "success",
	"data": { "groups": [] }
	}`,
	"/api/folders": "[]",
	"/api/recording-rules/writer": `{
	"id": "cojWep7Vz",
	"data_source_uid": "grafanacloud-prom",
	"remote_write_path": "/api/prom/push"
	}`,
	"/apis/banners.grafana.app/v0alpha1/namespaces/{stack}/announcement-banners": `{
	"kind": "AnnouncementBannerList",
	"apiVersion": "banners.grafana.app/v0alpha1",
	"metadata": {"resourceVersion": "29"}
	}`,
	}
	var blockJSONpost = map[string]string{
	"/api/frontend-metrics": "[]",
	"/api/search-v2": "[]",
	"/api/live/publish": "{}",
	"/api/ma/events": "null",
	}

It would allow to have this config close to the resource handler that requires it, instead of globally set by the server.