Allow network configuration to be overridden/merged with existing config

[jleeh]

For example:

NETWORK_SETTINGS=other-networks.yaml

When the above env var is set, it will result in the other-networks.yaml file to be deep merged into the networks.yaml file in the root of this repo.

[codecov-commenter]

Codecov Report

Merging #152 (0084ef2) into main (5a3b2a0) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #152   +/-   ##
=======================================
  Coverage   11.52%   11.52%           
=======================================
  Files          11       11           
  Lines        1380     1380           
=======================================
  Hits          159      159           
  Misses       1205     1205           
  Partials       16       16           

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