Add EnvYAML Support and Config Accessors

[PenguinBoi12]

Replaces yaml.safe_load with EnvYAML in load_from_file to support environment variable substitution in config files using ${VAR} syntax.

Also adds get() and __getitem__() methods to Config to allow structured access to config values beyond the standard Matrix connection fields, including support for custom sections.

Changes:

• Swap yaml.safe_load for EnvYAML in load_from_file
• Store raw config data in self._data for downstream access
• Add get(self, key: str, *, section: str | None = None, default: Any = None) for optional lookups
• Add __getitem__ for required lookups with KeyError on missing keys