Plugins: Refactor forward of cookies, OAuth token and header modifications by introducing client middlewares (#58132)

Adding support for backend plugin client middlewares. This allows headers in outgoing 
backend plugin and HTTP requests to be modified using client middlewares.

The following client middlewares added:
Forward cookies: Will forward incoming HTTP request Cookies to outgoing plugins.Client 
and HTTP requests if the datasource has enabled forwarding of cookies (keepCookies).
Forward OAuth token: Will set OAuth token headers on outgoing plugins.Client and HTTP 
requests if the datasource has enabled Forward OAuth Identity (oauthPassThru).
Clear auth headers: Will clear any outgoing HTTP headers that was part of the incoming 
HTTP request and used when authenticating to Grafana.
The current suggested way to register client middlewares is to have a separate package, 
pluginsintegration, responsible for bootstrap/instantiate the backend plugin client with 
middlewares and/or longer term bootstrap/instantiate plugin management. 

Fixes #54135
Related to #47734
Related to #57870
Related to #41623
Related to #57065

pkg/expr/nodes.go

@@ -42,12 +42,11 @@ type baseNode struct {
 }
 
 type rawNode struct {
-	RefID         string `json:"refId"`
-	Query         map[string]interface{}
-	QueryType     string
-	TimeRange     TimeRange
-	DataSource    *datasources.DataSource
-	QueryEnricher QueryDataRequestEnricher
+	RefID      string `json:"refId"`
+	Query      map[string]interface{}
+	QueryType  string
+	TimeRange  TimeRange
+	DataSource *datasources.DataSource
 }
 
 func (rn *rawNode) GetCommandType() (c CommandType, err error) {
@@ -140,9 +139,8 @@ const (
 // DSNode is a DPNode that holds a datasource request.
 type DSNode struct {
 	baseNode
-	query         json.RawMessage
-	datasource    *datasources.DataSource
-	queryEnricher QueryDataRequestEnricher
+	query      json.RawMessage
+	datasource *datasources.DataSource
 
 	orgID      int64
 	queryType  string
@@ -171,15 +169,14 @@ func (s *Service) buildDSNode(dp *simple.DirectedGraph, rn *rawNode, req *Reques
 			id:    dp.NewNode().ID(),
 			refID: rn.RefID,
 		},
-		orgID:         req.OrgId,
-		query:         json.RawMessage(encodedQuery),
-		queryType:     rn.QueryType,
-		intervalMS:    defaultIntervalMS,
-		maxDP:         defaultMaxDP,
-		timeRange:     rn.TimeRange,
-		request:       *req,
-		datasource:    rn.DataSource,
-		queryEnricher: rn.QueryEnricher,
+		orgID:      req.OrgId,
+		query:      json.RawMessage(encodedQuery),
+		queryType:  rn.QueryType,
+		intervalMS: defaultIntervalMS,
+		maxDP:      defaultMaxDP,
+		timeRange:  rn.TimeRange,
+		request:    *req,
+		datasource: rn.DataSource,
 	}
 
 	var floatIntervalMS float64
@@ -232,10 +229,6 @@ func (dn *DSNode) Execute(ctx context.Context, now time.Time, _ mathexp.Vars, s
 		Headers: dn.request.Headers,
 	}
 
-	if dn.queryEnricher != nil {
-		ctx = dn.queryEnricher(ctx, req)
-	}
-
 	resp, err := s.dataService.QueryData(ctx, req)
 	if err != nil {
 		return mathexp.Results{}, err