Add singbox core as a choice `in Settings

[ebkarimi]

v2rayNG is the best v2ray client for Android but it only uses Xray core. Please add sing box as a choice in the Settings. Sing box core is better in many situations than Xray.

[fransewoo]

+1，同求大佬更新可以支持sing_box内核切换。

[soraclub]

+1, 同求支持sing-box