Integrate Syslog Appender

[bconn98]

As brought up a couple times in the past #18 #26, there is a desire to use log4rs while writing to syslog. This capability could be utilized by applications running in Kubernetes in conjunction with ELK Stack. This issue can act as a sounding board / design approval for introduction of a new and complex appender that has alluded this crate for 8 years.

[bconn98]

Previous discussions in #18 brought forth the following suggestions.

• Base the implementation on Syslog appender #26
• Copy or call the implementation from the MIT licensed rust-syslog crate (relevant code probably less than 500 LOC)?
• Re-implement syslog
• Create a RELP crate and utilize it here

[bconn98]

It looks like version 6+ of rust-syslog has added integration with the log crate. This should help with integrating their solution I'm hoping. Further investigation is needed.

[bconn98]

After some further investigation into the rust-syslog project and considering how we can use their API. My gut feeling is that we should wrap their tool within the log4rs baseline. This reduces duplicate code & empowers us to take updates as both projects grow. We would of course need to add configuration capabilities as well.

1. Syslog Appender is a wrapper around rust-syslog
2. Config - new work
3. Encode - Similar to JSON, I purpose an extremely limited configuration interface
4. Filter - Limit to just the Rust log library supported log levels. Do not support critical, alert, or emergency.