chore(plugins): plugin system improvements

Summary

• update_plugin_tiers API call is split into service_append_tier and service_remove_tier calls
• now picoplugin expose tarantool-module from git-submodule
• plugin system improvements
  • add description field to _pico_plugin and _pico_service
  • plugin loading: filter files by its extensions
  • plugin loading: check possible service collisions
  • plugin loading: now plugin searching at {plugin_dir}/{plugin_name} path
  • now all errors between plugin and picodata wrapping with BoxError (this protects plugin caller from UAF)

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• Changes should be cherry-picked to 24.2: no
• Docs follow-up: docs#264 (closed), docs#238 (closed)