feat: add command line config overrides support

[YutongGu]

Description

This PR adds the ability to override configuration values from the command line without modifying the YAML config file.
This is useful for:

• Testing different configurations quickly
• Running multiple instances with slightly different settings
• Scripting and automation
• Overriding specific values in containerized environments

Usage

# Override a single field
go-librespot -c device_name=MyDevice

# Override nested fields using dot notation
go-librespot --conf server.address=0.0.0.0 --conf server.port=8080

# Multiple overrides
go-librespot -c device_type=speaker -c bitrate=320 -c credentials.type=interactive

# Mix with other flags
go-librespot --config_dir /custom/path -c zeroconf_enabled=true

Configuration Priority

Overrides are applied after all other configuration sources, giving them the highest priority:

1. -c/--conf overrides ← highest priority
2. Other command line flags
3. YAML config file (config.yaml / config.yml)
4. Default values ← lowest priority

Changes

• Added -c/--conf flag using StringArrayVarP to accept multiple override values
• Parses each override in field=value format
• Supports nested fields via dot notation (e.g., server.address, credentials.type)
• Includes validation for malformed overrides (missing =, empty field names)

Error Handling

The following cases produce clear error messages:

• Missing = separator: invalid config override format: fieldvalue (expected field=value)
• Empty field name: invalid config override: empty field name in =value

[devgianlu]

Thanks!