IDForwarding: Add auth hook to generate id token (#75555)

* AuthN: Move identity struct to its own file

* IDForwarding: Add IDToken property to usr and identity structs and add GetIDToken to requester interface

* Inject IDService into background services

* IDForwarding: Register post auth hook when feature toggle is enabled

pkg/services/user/identity.go

@@ -27,6 +27,9 @@ type SignedInUser struct {
 	Teams            []int64
 	// Permissions grouped by orgID and actions
 	Permissions map[int64]map[string][]string `json:"-"`
+	// IDToken is a signed token representing the identity that can be forwarded to plugins and external services.
+	// Will only be set when featuremgmt.FlagIdForwarding is enabled.
+	IDToken string `json:"-" xorm:"-"`
 }
 
 func (u *SignedInUser) ShouldUpdateLastSeenAt() bool {
@@ -210,3 +213,7 @@ func (u *SignedInUser) GetDisplayName() string {
 func (u *SignedInUser) GetAuthenticatedBy() string {
 	return u.AuthenticatedBy
 }
+
+func (u *SignedInUser) GetIDToken() string {
+	return u.IDToken
+}