feat(router): add support for custom service URLs

[piksel]

This will allow services to opt in to supporting the service+custom URL schema, which can be used for simplifying the setup in basic scenarios, or to support multiple URL formats.

Example usage by teams in #117 :
User get a webhook URL from Teams, and simply prepends teams+ to the URL to make it a shoutrrr URL:
teams+https://outlook.office.com/webhook/12345....

[codecov]

Codecov Report

Merging #125 (f058fb6) into main (6e64c70) will increase coverage by 1.81%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main     #125      +/-   ##
==========================================
+ Coverage   58.15%   59.96%   +1.81%     
==========================================
  Files          55       55              
  Lines        1171     1184      +13     
==========================================
+ Hits          681      710      +29     
+ Misses        440      424      -16     
  Partials       50       50              

Impacted Files	Coverage Δ
pkg/router/router.go	67.46% <50.00%> (+7.46%)	⬆️
pkg/router/servicemap.go	100.00% <0.00%> (+93.75%)	⬆️

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