fix: syslog plugin doesn't work

[jiangfucheng]

Description

Fixes #9419

1.Convert the log message to RFC5424 format before sending it to the syslog server.
2.RFC5424 is “The Syslog Protocol", not only used for sls-logger. So, I relocated the file rfc5424.lua from apisix/plugins/slslog to apisix/syslog

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[monkeyDluffy6017]

@Sn0rt please help to review

[Sn0rt]

@jiangfucheng thx your PR.

can you add a test set to verify the syslog plugin is really working?

I think https://github.com/apache/apisix/blob/master/t/plugin/tcp-logger.t#L466 is helpful for you.

I think the below config file of vector can help you save more time.

file path ci/pod/vector/vector.toml

[sources.log-from-syslog-tcp]
type = "syslog"
address = "0.0.0.0:5050"
mode = "tcp"

[sources.log-from-syslog-udp]
type = "syslog"
address = "0.0.0.0:5050"
host_key = "host"
mode = "udp"
port_key = "port"
shutdown_timeout_secs = 30
socket_file_mode = 511

[sinks.log-2-console]
inputs = [ "log-from-tcp",  "log-from-tls", "log-from-syslog-tcp", "log-from-syslog-udp"]
type = "console"
encoding.codec = "json"

[sinks.log-2-syslog-tcp-file]
inputs = [ "log-from-syslog-tcp" ]
type = "file"
encoding.codec = "text"
path = "/etc/vector/syslog-tcp.log"

[sinks.log-2-syslog-udp-file]
inputs = [ "log-from-syslog-udp" ]
type = "file"
encoding.codec = "text"
path = "/etc/vector/syslog-udp.log"

and some new test set at tcp-logger.lua to verify the syslog is real working.

[jiangfucheng]

@Sn0rt Updated, please review again, thanks.

[Sn0rt]

LGTM @monkeyDluffy6017 pls approve CI.

[Sn0rt]

@shreemaan-abhishek your commit has been duplicated here

[monkeyDluffy6017]

@jiangfucheng Could you merge the master branch, some ci error are fixed