feat(fields): add instance domain (#9000)

# Which Problems Are Solved

Instance domains are only computed on read side. This can cause missing
domains if calls are executed shortly after a instance domain (or
instance) was added.

# How the Problems Are Solved

The instance domain is added to the fields table which is filled on
command side.

# Additional Changes

- added setup step to compute instance domains
- instance by host uses fields table instead of instance_domains table

# Additional Context

- part of https://github.com/zitadel/zitadel/issues/8999

cmd/setup/config.go

@@ -128,6 +128,7 @@ type Steps struct {
 	s38BackChannelLogoutNotificationStart   *BackChannelLogoutNotificationStart
 	s40InitPushFunc                         *InitPushFunc
 	s39DeleteStaleOrgFields                 *DeleteStaleOrgFields
+	s41FillFieldsForInstanceDomains         *FillFieldsForInstanceDomains
 }
 
 func MustNewSteps(v *viper.Viper) *Steps {