Add dist sink support

[nextsilicon-itay-bookstein]

Hi again!

I implemented support for dist sinks (including verification that no circular references are made), and we're already using them. Was wondering whether you'd be interested. I saw that they were documented as 'not part of the use case', but I figured they might be useful for others some time or other, for grouping/aggregation within the configuration file :)

I'm a bit dissatisfied with the two dynamic casts, but alas, there is no common base class for both template instantiations that would allow me to call the relevant API, and I didn't want to propagate 'redundant' information through parameters.

[codecov[bot]]

Codecov Report

Merging #69 (cb3e807) into master (a991861) will increase coverage by 0.19%. The diff coverage is 92.10%.

@@            Coverage Diff             @@
##           master      #69      +/-   ##
==========================================
+ Coverage   88.16%   88.35%   +0.19%     
==========================================
  Files           4        4              
  Lines         566      601      +35     
==========================================
+ Hits          499      531      +32     
- Misses         67       70       +3     

Impacted Files	Coverage Δ
include/spdlog_setup/details/conf_impl.h	90.26% <92.10%> (+0.10%)	:arrow_up:

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