Support key values from slf4j's fluent api

[mmienko]

This should address issue #946.
The configurations are the same as the configurations for MDC.

[ferhtaydn]

LGTM.

@osigida can you merge if there's nothing left to address?

[codecov]

Codecov Report

Merging #954 (07314e5) into main (af9f826) will decrease coverage by 0.40%.
The diff coverage is 54.38%.

@@             Coverage Diff              @@
##               main     #954      +/-   ##
============================================
- Coverage     72.05%   71.66%   -0.40%     
- Complexity     1353     1386      +33     
============================================
  Files           173      174       +1     
  Lines          4999     5113     +114     
  Branches        517      528      +11     
============================================
+ Hits           3602     3664      +62     
- Misses         1148     1189      +41     
- Partials        249      260      +11     

Impacted Files	Coverage Δ
...va/net/logstash/logback/layout/LogstashLayout.java	0.00% <0.00%> (ø)
...n/java/net/logstash/logback/LogstashFormatter.java	53.00% <32.35%> (-4.23%)	⬇️
.../net/logstash/logback/encoder/LogstashEncoder.java	55.55% <53.33%> (-0.51%)	⬇️
...posite/loggingevent/KeyValuePairsJsonProvider.java	84.44% <84.44%> (ø)
...posite/loggingevent/LoggingEventJsonProviders.java	87.75% <100.00%> (+0.52%)	⬆️
...ogstash/logback/fieldnames/LogstashFieldNames.java	93.44% <100.00%> (+0.33%)	⬆️

[philsttr]

Thank you so much for the contribution and reviews!