feat: specify login UI version on instance and apps (#9071)

# Which Problems Are Solved

To be able to migrate or test the new login UI, admins might want to
(temporarily) switch individual apps.
At a later point admin might want to make sure all applications use the
new login UI.

# How the Problems Are Solved

- Added a feature flag `` on instance level to require all apps to use
the new login and provide an optional base url.
- if the flag is enabled, all (OIDC) applications will automatically use
the v2 login.
  - if disabled, applications can decide based on their configuration
- Added an option on OIDC apps to use the new login UI and an optional
base url.
- Removed the requirement to use `x-zitadel-login-client` to be
redirected to the login V2 and retrieve created authrequest and link
them to SSO sessions.
- Added a new "IAM_LOGIN_CLIENT" role to allow management of users,
sessions, grants and more without `x-zitadel-login-client`.

# Additional Changes

None

# Additional Context

closes https://github.com/zitadel/zitadel/issues/8702

internal/feature/feature.go

@@ -1,6 +1,9 @@
 package feature
 
-import "slices"
+import (
+	"net/url"
+	"slices"
+)
 
 //go:generate enumer -type Key -transform snake -trimprefix Key
 type Key int
@@ -19,6 +22,7 @@ const (
 	KeyOIDCSingleV1SessionTermination
 	KeyDisableUserTokenEvent
 	KeyEnableBackChannelLogout
+	KeyLoginV2
 )
 
 //go:generate enumer -type Level -transform snake -trimprefix Level
@@ -47,6 +51,7 @@ type Features struct {
 	OIDCSingleV1SessionTermination  bool                      `json:"oidc_single_v1_session_termination,omitempty"`
 	DisableUserTokenEvent           bool                      `json:"disable_user_token_event,omitempty"`
 	EnableBackChannelLogout         bool                      `json:"enable_back_channel_logout,omitempty"`
+	LoginV2                         LoginV2                   `json:"login_v2,omitempty"`
 }
 
 type ImprovedPerformanceType int32
@@ -63,3 +68,8 @@ const (
 func (f Features) ShouldUseImprovedPerformance(typ ImprovedPerformanceType) bool {
 	return slices.Contains(f.ImprovedPerformance, typ)
 }
+
+type LoginV2 struct {
+	Required bool     `json:"required,omitempty"`
+	BaseURI  *url.URL `json:"base_uri,omitempty"`
+}