💄CLI interface improvements

[parfeniukink]

Summary

• The click CLI interface is tested with a bunch of unit tests
• The main function validation is added.
• OpenAIBackend initializer parameters are optimized
  • target, host, and port parameters usage is simplified
• openai.NotFound available models error is handled
• SerializableFileType renamed to SerializableFileExtension
• SerializableFileExtension now inherits str to simplify usage, since this Enum class is mostly used to work with strings.
• rate_type_to_load_gen_mode renamed to RATE_TYPE_TO_LOAD_GEN_MODE_MAPPER
• rate_type_to_profile_mode renamed to RATE_TYPE_TO_PROFILE_MODE_MAPPER
• CLI parameters are renamed:
  • --num-seconds -> --max-seconds
  • --num-requests -> --max-requests
• path removed from CLI arguments since it is not used
• .env GUIDELLM prefix is fixed
• Unused comments, settings, and code are removed
• Logger default unit test uses the injected logging settings object
• Module backend.openai has _base_url renamed to the base_url
• In OpenAIBackend.make_request, the GenerativeResponse always counts output_tokens with self._token_count
• SerializableFileExtensions is replaced with pure Python strings