New port: sysutils/flowgger

Flowgger is a fast, simple and lightweight data collector. It reads
log entries over a given protocol, extracts them, decodes them using a
given format, re-encodes them into a different format, and
asynchronously pushes the result into a remote data store.

Flowgger is designed to be:

• Paranoid: it carefully validates input data to prevent injection of malformed/incomplete records down the chain.
• Safe: written in Rust, without any unsafe code.
• Fast: even though messages are systematically parsed and validated, Flowgger is orders of magnitude faster than Logstash and Fluentd.
• Standalone: it comes as a single executable file, and doesn't require a JVM.

Flowgger supports common input types: stdin, UDP, TCP, TLS and Redis,
as well as multiple input formats: JSON (GELF), LTSV, Cap'n Proto and
RFC5424. Normalized messages can be sent to Kafka, Graylog, to
downstream Flowgger servers, or to other log collectors for further
processing.

WWW: https://github.com/jedisct1/flowgger

Approved by: mat (mentor)
Differential Revision: https://reviews.freebsd.org/D11162