feat: backend support for creating and storing service accounts (#22698)

Add is_service_account column to users table with CHECK constraints
enforcing login_type='none' and empty email for service accounts.
Update user creation API to validate service account constraints.

Related to:
https://linear.app/codercom/issue/PLAT-27/feat-backend-support-for-creating-and-storing-service-accounts

coderd/database/models.go

@@ -4877,6 +4877,8 @@ type User struct {
 	OneTimePasscodeExpiresAt sql.NullTime `db:"one_time_passcode_expires_at" json:"one_time_passcode_expires_at"`
 	// Determines if a user is a system user, and therefore cannot login or perform normal actions
 	IsSystem bool `db:"is_system" json:"is_system"`
+	// Determines if a user is an admin-managed account that cannot login
+	IsServiceAccount bool `db:"is_service_account" json:"is_service_account"`
 }
 
 type UserConfig struct {