Support loading configuration from both YAML files and env vars

[lippserd]

This PR introduces config.Load() that supports configuration loading in three scenarios:

1. Load configuration solely from YAML files when no relevant environment variables are set.
2. Combine YAML file and environment variable configurations, allowing environment variables to supplement or override possible incomplete YAML configurations.
3. Load entirely from environment variables if the default YAML config file is absent and no specific config path is provided by the user.

config.FromYAMLFile() is still called first but continuation with config.FromEnv() is allowed by handling:

• ErrInvalidConfiguration: The configuration may be incomplete and will be revalidated in config.FromEnv().
• Non-existent file errors: If no explicit config path is set, fallback to environment variables is allowed.

config.FromEnv() is called regardless of the outcome from config.FromYAMLFile(). If no environment variables are set, configuration relies entirely on YAML. Otherwise, environment variables can supplement, override YAML settings, or serve as the sole source. config.FromEnv() also includes validation, ensuring completeness after considering both sources.

Possible alternative implementations:

• If the config path is the default, os.Stat() could be used before calling config.FromYAMLFile(), rather than handling non-existent file errors. This approach would split the logic into two sections and add an additional if block.
• Don't call Validate() in the config package automatically. Instead, require it to be called manually. I appreciate that library-wise, both config.FromYAMLFile() and config.FromEnv() include validation allowing them to be used without needing an additional function call on their own. When combining them, I think it's straightforward to use errors.Is() to check for ErrInvalidConfiguration, i.e. errors from Validate().

requires #83