feat: initial RFC3164 implementation

[cypherbits]

Fixes: #5

Initial RFC3164 added as a fallback when parsing RFC5424 fails.

I think we could add a env variable like SLOGGO_PARSE_FORMAT with values like:
"auto": automatic parsing, tries all rfc.
"RFC5424": tries just RFC5424.
"RFC3164": tries just RFC3164.

[nicolasbeauvais]

Thank you @cypherbits this is a good addition to Sloggo,

Could you add a SLOGGO_LOG_FORMAT env format as you suggested with the three options:

• auto
• RFC5424
• RFC3164

[cypherbits]

1. Added SLOGGO_LOG_FORMAT with these 3 options.
2. Added a startup log for the config that shows like this:

2025/11/26 17:58:05 Sloggo version: dev
2025/11/26 17:58:05 Config: listeners=[tcp udp] udp_port=5514 tcp_port=6514 api_port=8080
2025/11/26 17:58:05 Config: log_format=rfc3164 debug=true retention_minutes=43200