AuthN: Add auth proxy client (#61555)

* AuthN: set up boilerplate for proxy client

* AuthN: Implement Test for proxy client

* AuthN: parse accept list in constructor

* AuthN: add proxy client interface

* AuthN: handle error

* AuthN: Implement the proxy client interface for ldap

* AuthN: change reciever name

* AuthN: add grafana as a proxy client

* AuthN: for error returned

* AuthN: add tests for grafana proxy auth

* AuthN: swap order of grafan and ldap auth

* AuthN: Parse additional proxy headers in proxy client and pass down

pkg/services/authn/authn.go

@@ -25,6 +25,7 @@ const (
 	ClientRender    = "auth.client.render"
 	ClientSession   = "auth.client.session"
 	ClientForm      = "auth.client.form"
+	ClientProxy     = "auth.client.proxy"
 )
 
 const (
@@ -72,6 +73,10 @@ type PasswordClient interface {
 	AuthenticatePassword(ctx context.Context, r *Request, username, password string) (*Identity, error)
 }
 
+type ProxyClient interface {
+	AuthenticateProxy(ctx context.Context, r *Request, username string, additional map[string]string) (*Identity, error)
+}
+
 type Request struct {
 	// OrgID will be populated by authn.Service
 	OrgID int64