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]

Codecov Report

❌ Patch coverage is 92.10526% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.35%. Comparing base (a991861) to head (cb3e807).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
include/spdlog_setup/details/conf_impl.h	92.10%	3 Missing ⚠️

Additional details and impacted files

@@            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     

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