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/services/oauthtoken/oauth_token.go

@@ -79,7 +79,7 @@ func (o *Service) GetCurrentOAuthToken(ctx context.Context, usr *user.SignedInUs
 
 // IsOAuthPassThruEnabled returns true if Forward OAuth Identity (oauthPassThru) is enabled for the provided data source.
 func (o *Service) IsOAuthPassThruEnabled(ds *datasources.DataSource) bool {
-	return ds.JsonData != nil && ds.JsonData.Get("oauthPassThru").MustBool()
+	return IsOAuthPassThruEnabled(ds)
 }
 
 // HasOAuthEntry returns true and the UserAuth object when OAuth info exists for the specified User
@@ -204,6 +204,11 @@ func (o *Service) tryGetOrRefreshAccessToken(ctx context.Context, usr *models.Us
 	return token, nil
 }
 
+// IsOAuthPassThruEnabled returns true if Forward OAuth Identity (oauthPassThru) is enabled for the provided data source.
+func IsOAuthPassThruEnabled(ds *datasources.DataSource) bool {
+	return ds.JsonData != nil && ds.JsonData.Get("oauthPassThru").MustBool()
+}
+
 // tokensEq checks for OAuth2 token equivalence given the fields of the struct Grafana is interested in
 func tokensEq(t1, t2 *oauth2.Token) bool {
 	return t1.AccessToken == t2.AccessToken &&

pkg/services/oauthtoken/oauthtokentest/oauthtokentest.go

@@ -0,0 +1,41 @@
+package oauthtokentest
+
+import (
+	"context"
+
+	"github.com/grafana/grafana/pkg/models"
+	"github.com/grafana/grafana/pkg/services/datasources"
+	"github.com/grafana/grafana/pkg/services/oauthtoken"
+	"github.com/grafana/grafana/pkg/services/user"
+	"golang.org/x/oauth2"
+)
+
+// Service an OAuth token service suitable for tests.
+type Service struct {
+	Token *oauth2.Token
+}
+
+// ProvideService provides an OAuth token service suitable for tests.
+func ProvideService() *Service {
+	return &Service{}
+}
+
+func (s *Service) GetCurrentOAuthToken(context.Context, *user.SignedInUser) *oauth2.Token {
+	return s.Token
+}
+
+func (s *Service) IsOAuthPassThruEnabled(ds *datasources.DataSource) bool {
+	return oauthtoken.IsOAuthPassThruEnabled(ds)
+}
+
+func (s *Service) HasOAuthEntry(context.Context, *user.SignedInUser) (*models.UserAuth, bool, error) {
+	return nil, false, nil
+}
+
+func (s *Service) TryTokenRefresh(context.Context, *models.UserAuth) error {
+	return nil
+}
+
+func (s *Service) InvalidateOAuthTokens(context.Context, *models.UserAuth) error {
+	return nil
+}