draft: New `allow_private_endpoints` stanza

[Tapanito]

High Level Overview of Change

Context of Change

Adds a new config stanza allow_private_endpoints. The new config option allows servers to accept and advertise open slots for servers with private IP addresses.

Type of Change

• [] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Refactor (non-breaking change that only restructures code)
• [ ] Performance (increase or change in throughput and/or latency)
• [x] Tests (you added tests for code that already exists, or your new feature included in this PR)
• [ ] Documentation update
• [ ] Chore (no impact to binary, e.g. .gitignore, formatting, dropping support for older tooling)
• [ ] Release

API Impact

• [ ] Public API: New feature (new methods and/or new fields)
• [ ] Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• [ ] libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• [ ] Peer protocol change (must be backward compatible or bump the peer protocol version)

[codecov[bot]]

Codecov Report

:x: Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review. :white_check_mark: Project coverage is 78.1%. Comparing base (c17676a) to head (b4fb904). :warning: Report is 111 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/xrpld/peerfinder/detail/PeerfinderConfig.cpp	66.7%	1 Missing :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #5309   +/-   ##
=======================================
  Coverage     78.1%   78.1%           
=======================================
  Files          790     790           
  Lines        67907   67916    +9     
  Branches      8225    8218    -7     
=======================================
+ Hits         53032   53056   +24     
+ Misses       14875   14860   -15     

Files with missing lines	Coverage Δ
src/xrpld/core/Config.h	85.7% <ø> (ø)
src/xrpld/core/ConfigSections.h	100.0% <ø> (ø)
src/xrpld/core/detail/Config.cpp	75.3% <100.0%> (+0.1%)	:arrow_up:
src/xrpld/peerfinder/PeerfinderManager.h	50.0% <ø> (ø)
src/xrpld/peerfinder/detail/Logic.h	55.1% <100.0%> (+5.1%)	:arrow_up:
src/xrpld/peerfinder/detail/PeerfinderConfig.cpp	82.8% <66.7%> (-0.9%)	:arrow_down:

... and 2 files with indirect coverage changes

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[vlntb]

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