refactor: separate the logic of `inbound fallback` so that it can be reused

[Grvzard]

works fine in deployment

[Fangliding]

~~Can you provide a detailed explanation of the changes?~~ It looks like integrated the separate fallback implementation of trojan and vless together Now we can easily add fallback in other protocols (~~is it really exits?~~)

[yuhan6665]

Couple of thought:

1. Write Everything Twice, then de-duplicate if more than three times
2. Trojan is sunsetting, not worth much refactor So I think current code is ok, thanks for your effort though

[RPRX]

如 @yuhan6665 所言，这部分代码其实没必要动，如果觉得代码重复我们可以删掉 Trojan 的 fallbacks 支持，但目前来说没必要动

[Grvzard]

1. Write Everything Twice, then de-duplicate if more than three times

Totally agree.

  I think current code is ok

I think xray-core is a tool for creating new protocols, so these changes are made for my convenience. It's okay if the upstream doesn't need them.

[RPRX]

总之还是感谢 PR