chore(BUX-426): BroadcastClientAPIs as struct

[chris-4chain]

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• 📖 I created my PR using provided : CODE_STANDARDS
• 📖 I have read the short Code of Conduct: CODE_OF_CONDUCT
• 🏠 I tested my changes locally.
• ✅ I have provided tests for my changes.
• 📝 I have used conventional commits.
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Previous config for nodes/broadcast_client_apis looked like this:

nodes:
  broadcast_client_apis: ["URL", "URL2|TOKEN2"]

where the vertical bar | was a separator.

In this PR I'm proposing a more standard approach using structs, like this:

nodes:
  broadcast_client_apis:
    - url: https://arc.gorillapool.io

    - url: https://tapi.taal.com/arc
      token: <TOKEN>

nodes:
  broadcast_client_apis:
    - url: https://arc.gorillapool.io
    - url: https://some_open_arc.com

nodes:
  broadcast_client_apis:
    - url: https://arc.gorillapool.io
      token:

nodes:
  broadcast_client_apis: []

The token is optional.

[mergify]

Welcome to our open-source project @chris-4chain! 💘

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (aa41772) 39.02% compared to head (d7633db) 39.16%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #386      +/-   ##
==========================================
+ Coverage   39.02%   39.16%   +0.14%     
==========================================
  Files          44       44              
  Lines        1576     1560      -16     
==========================================
- Hits          615      611       -4     
+ Misses        928      917      -11     
+ Partials       33       32       -1     

Flag	Coverage Δ
unittests	39.16% <16.66%> (+0.14%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
config/config.go	100.00% <ø> (ø)
config/defaults.go	100.00% <100.00%> (ø)
config/services.go	45.48% <0.00%> (+1.07%)	⬆️

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