feat(service): add generic webhook service

[piksel]

This adds a custom webhook service that just posts the message to the URL that corresponds to the URL.

To use it with another format, set the template query field (generic://example.com/api/webhook?template=TEMPLATE_ID or pass it as a param to Send:

err = service.Send("Message", &types.Params{"template": "TEMPLATE_ID"})

The templates are loaded into the service by using either

err := service.SetTemplateString("TEMPLATE_ID", `{{.message}}`)

or

err := service.SetTemplateFile("TEMPLATE_ID", "/path/to/template.file")

Where TEMPLATE_ID can be anything.

[codecov]

Codecov Report

Merging #144 (58ed40d) into main (469140f) will increase coverage by 4.33%.
The diff coverage is 85.18%.

@@            Coverage Diff             @@
##             main     #144      +/-   ##
==========================================
+ Coverage   63.98%   68.32%   +4.33%     
==========================================
  Files          70       72       +2     
  Lines        1999     2096      +97     
==========================================
+ Hits         1279     1432     +153     
+ Misses        606      547      -59     
- Partials      114      117       +3     

Impacted Files	Coverage Δ
pkg/format/format_query.go	78.12% <79.16%> (+78.12%)	⬆️
pkg/format/prop_key_resolver.go	76.92% <80.00%> (+76.92%)	⬆️
pkg/services/generic/generic.go	83.78% <83.78%> (ø)
pkg/services/generic/generic_config.go	93.54% <93.54%> (ø)
pkg/router/servicemap.go	100.00% <100.00%> (ø)
pkg/format/node.go	91.17% <0.00%> (+0.73%)	⬆️
pkg/format/formatter.go	66.66% <0.00%> (+3.33%)	⬆️
pkg/format/enum_formatter.go	70.00% <0.00%> (+10.00%)	⬆️
... and 3 more

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