[Feature Request] Listen内的Fallback增加xver字段

[SakuraSakuraSakuraChan]

Describe the solution you'd like 【具体描述一下你想要的新功能，或者你的新想法】
A clear and concise description of what you want to happen.

现在80和443都交给VS了，非代理流量会回落给nginx。最近有了看nginx日志的需求，但是IP部分都是 unix(或127.0.0.1)。根据 #51 和 https://github.com/e1732a364fed/v2ray_simple/blob/main/examples/vlesss.server.toml ，需要在 [[fallback]] 内添加 xver 参数。但是问题来了，我的fallback是写在 [[listen]] 里的，如果在 [[listen]] 里写xver，那就是另一个意思了，因此希望增加一个 Listen里的专用字段，表示Listen内Fallback使用的xver

那为什么不专门写个 [[fallback]] 捏？我还真写了，但是回落不成功，大概属于bug或者我配置问题，就不写在这个Feature Request的主楼里了。配置放在下边，大佬有时间的话看一眼没时间就算了

[SakuraSakuraSakuraChan]

[[listen]]
protocol = "trojans"
ip = "0.0.0.0"
host = "fuck.gfw.com"
port = 443
fallback = "/dev/shm/http.socket"
adv = "grpc"
path = "fuckgfw"

[[listen]]
protocol = "vmess"
ip = "0.0.0.0"
port = 80
fallback = "/dev/shm/vs.socket"
adv = "ws"
path = "/fuckgfw"
early = true

[[fallback]]
from = ["Osa_Trojan"]
alpn = ["http/1.1"]
dest = "/dev/shm/h2c.socket"

[[fallback]]
from = ["Osa_Trojan"]
alpn = ["h2"]
dest = "/dev/shm/h2c.socket"

[[dial]]
protocol = "direct"

上边是我用了很久的配置，各种回落都没问题，现在为了增加80端口回落的Proxy protocol，把配置改为了下边的

[[listen]]
protocol = "trojans"
ip = "0.0.0.0"
host = "fuck.gfw.com"
port = 443
fallback = "/dev/shm/http.socket"
adv = "grpc"
path = "fuckgfw"

[[listen]]
tag = "VM"
protocol = "vmess"
ip = "0.0.0.0"
port = 80
adv = "ws"
path = "/fuckgfw"
early = true

[[fallback]]
from = ["VM"]
dest = "/dev/shm/VS.socket"
xver = 1

[[fallback]]
from = ["Osa_Trojan"]
alpn = ["http/1.1"]
dest = "/dev/shm/h2c.socket"

[[fallback]]
from = ["Osa_Trojan"]
alpn = ["h2"]
dest = "/dev/shm/h2c.socket"

[[dial]]
protocol = "direct"

结果80端口就回落不到nginx了，但是443的回落正常。试过注释掉xver也一样