JSON log output

[markphelps]

Fixes #992

1. Adds support for encoding log output as JSON using either FLIPT_LOG_ENCODING env var or setting log.encoding via yaml
2. Only prints banner and api/ui addrs as text if we're running in console mode

/cc @GeorgeMac

TODO

• Update docs

[codecov-commenter]

Codecov Report

Merging #1027 (01cb0b0) into main (4914fdf) will decrease coverage by 0.04%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##             main    #1027      +/-   ##
==========================================
- Coverage   81.26%   81.21%   -0.05%     
==========================================
  Files          17       17              
  Lines        1799     1805       +6     
==========================================
+ Hits         1462     1466       +4     
- Misses        263      265       +2     
  Partials       74       74              

Impacted Files	Coverage Δ
config/config.go	89.14% <71.42%> (-0.54%)	⬇️

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[GeorgeMac]

@markphelps nice. Yeah, might be worth dropping the banner when the encoding is JSON.

As you say, the JSON encoder is likely to imply the output of the logging here is machine-readable.
However, piping it to jq or down to a central logging system will likely break that with the banner.

[GeorgeMac]

Looks great. Couple take it or leave it suggestions.