fix: filter out external subnets

[sebastiendan]

Description

This PR filters out external subnets so that only Topos and Incal are usable with the faucet.

PR Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added or updated tests that comprehensively prove my change is effective or that my feature works

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (13513b2) 54.85% compared to head (5cb9164) 54.90%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #20      +/-   ##
==========================================
+ Coverage   54.85%   54.90%   +0.04%     
==========================================
  Files          33       33              
  Lines         937      938       +1     
  Branches       57       57              
==========================================
+ Hits          514      515       +1     
  Misses        414      414              
  Partials        9        9              

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