Idea: tt support Tarantool 3.* apps without instances.yml

[psergee]

Currently, tt always requires a instances.yml file for multi-instance applications. This is fine for cartridge-based apps, but it can be inconvenient for Tarantool 3.x apps development. A Tarantool 3.x app already has a centralized configuration that contains a list of available instances, and all these instances should be listed in the instances.yml file. The idea is to use all the instances listed in the configuration, even if the instances.yml file is missing. If the user runs tt start, and there are too many instances in the config, tt will prompt for confirmation. During application distribution, the instances.yml could be generated to specify which instances to run.