feat: make syslog logging useable

[maxmoehl]

• Read the Contributing document.

Summary

This PR corrects and improves the direct-to-syslog logging of access logs for Gorouter.

• does not log an empty new line when writing to syslog
• allows the operator to configure truncation on syslog, as syslog servers usually have a message limit
• significant performance improvement to the syslog implementation

The syslog implementation in Go has not been maintained by the Go authors since around 2015. While there are forks of the syslog code, all of them are also dormant. This PR inlines and significantly rewrites the syslog implementation, focusing on the following performance improvements:

• No use of the expensive fmt.Sprintf
• Determine all "static" fields (e.g. the PID) on initialization, not per message
• Correct handling of UTF-8 BOM according to RFC5424

Limitations:

• Only TCP and UDP are supported, as described in the routing-release property for the syslog network. All other means (unix sockets, etc.) are not supported.

Backward Compatibility

Breaking Change? Yes

• This PR moves the syslog implementation from the BSD-style syslog format, which is not standardised, to the RFC5424 format.

[maxmoehl]

I forgot to update the breaking change indicator after pushing my latest change. This is technically breaking because we move from the BSD-style syslog format which is not standardised to the RFC5424 format. I'm not sure how man people actually use the syslog implementation today, over UDP each log line also inserts an empty log line due to the (previously dedicated) call to Write. Over TCP it did probably work, however, it caused gorouter to perform worse as the old implementation did unnecessary syscalls for each log line written and the formatting was done using fmt.

[ameowlia]

Discussion in slack here: https://cloudfoundry.slack.com/archives/C02HNDJB31R/p1732140514501099

[maxmoehl]

The old format looks something like this:

<34>Oct 11 22:14:15 mymachine MESSAGE

and the new format is something like this:

<34>1 2003-10-11T22:14:15.003Z mymachine.example.com su - ID47 - \ufeffMESSAGE

[maxmoehl]

For testing use this config:

logging.syslog_addr: 127.0.0.1:514
logging.syslog_network: udp
enable_access_log_streaming: true
write_access_logs_locally: false

[maxmoehl]

I've also opened cloudfoundry/routing-release#446