* Add SelfHostedOrganizationSignUpCommand for organization sign-up process
Method extracted from OrganizationService
* Register SelfHostedOrganizationSignUpCommand for dependency injection
* Add unit tests for SelfHostedOrganizationSignUpCommand
* Refactor SelfHostedOrganizationLicensesController to use ISelfHostedOrganizationSignUpCommand
* Remove SignUpAsync method and related validation from IOrganizationService and OrganizationService
* Move ISelfHostedOrganizationSignUpCommand into a separate file and update references
* Enable null safety in SelfHostedOrganizationSignUpCommand and update ISelfHostedOrganizationSignUpCommand interface to reflect nullable types for organizationUser and collectionName.
