Implement pyproject.toml support for Locust configuration

[alzex3]

Issue Addressed:

This PR implements support for TOML files as configuration files and sets pyproject.toml as the default. It addresses the #2298 issue.

Implementation Details:

1. Used the built-in CompositeConfigParser with TomlConfigParser and DefaultConfigFileParser, which allow the configargparse lib to automatically parse options from defined .conf and .toml files.
2. Added pyproject.toml to DEFAULT_CONFIG_FILES.
3. Introduced a new project dependency - toml, which is required for the configargparse TOML parser to work.
4. Added tests to check the correct parsing of .conf and .toml configuration files.
5. Reworked the Configuration File documentation section.

[alzex3]

@cyberw Hello! I've replaced the toml lib and added a custom parser.

[cyberw]

Awesome.