Allow to set multiple default service URLs.

[Eskibear]

This PR enables the feature to set multiple service URL in settings.

1. Type of the config is changed to string or array of strings

• if string, fallback to previous behavior.
• if array, it first require users to pick on from the list.

2. About the code changes, as different services has different results, now I added a new class ServiceManager to handle stuffs like getting available versions, starters, etc.

[Eskibear]

in 3204e52 :

a string: use the string as service url directly
an empty array : fallback to use the official service
an array with single element: use the element as service url directly
an array with 2+ elements: let users select one.