Add multiple valid config file locations

[pederhan]

The application should look for the config file in multiple different locations, such as:

• <platformdirs directory>/config.toml (current)
• ./config.toml
• /etc/harbor_cli/config.toml
• /usr/local/etc/harbor_cli/config.toml

We'll have to decide what the priority should be. Configs in user directories (default appdir location, $(pwd) and /usr/local/etc) should definitely take priority over site config dirs (/etc).

Possible implementation

# dirs.py
CONFIG_DIR = Path(_PLATFORM_DIR.user_config_dir)
SITE_CONFIG_DIR = Path(_PLATFORM_DIR.site_config_dir)

# config.py

CONFIG_FILENAME = "config.toml"
DEFAULT_CONFIG_FILE = CONFIG_DIR / "config.toml"

class HarborCLIConfig(BaseModel):
    # ...

    @classmethod
    def from_file(
        cls, config_file: Path | None = None, create: bool = False
    ) -> HarborCLIConfig:
        if config_file is None:
            config_file = find_config_file()
   # ...


def find_config_file() -> Path:
    """Attempts to find a config file. Falls back on the default config file location if a config file
    cannot be found in any of the expected locations."""
    for d in [Path(""), CONFIG_DIR, SITE_CONFIG_DIR]:
        p = d / CONFIG_FILENAME
        if p.exists():
            return p
    return DEFAULT_CONFIG_FILE # fall back on default (may not exist yet)