feat: Add GET /instance endpoint, which returns some basic information about the current session

[Twinki14]

• Endpoint addition, main intention is to provide an alternative to using the web-api for basic information about the session including device_id, device_name, and device_type
• device_id, device_name,device_type, country_code, and preferred_locale

Example request:

{
    "device_id": "22ed0b3e74ad2001d16b7ea751fc58bb7a1ae686",
    "device_name": "librespot-java",
    "device_type": "COMPUTER",
    "country_code": "US",
    "preferred_locale": "en"
}