NUT-06: add icon URL

[callebtc]

Implements cashubtc/nuts#130

[prusnak]

update to reflect comment cashubtc/nuts#130 (comment)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.10%. Comparing base (8675745) to head (d95f680).
Report is 9 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (8675745) and HEAD (d95f680). Click for more details.

HEAD has 6 uploads less than BASE

Flag	BASE (8675745)	HEAD (d95f680)
	9	3

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #604      +/-   ##
==========================================
- Coverage   69.02%   59.10%   -9.92%     
==========================================
  Files          92       97       +5     
  Lines        8674    10388    +1714     
==========================================
+ Hits         5987     6140     +153     
- Misses       2687     4248    +1561     

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