feat(slack): add bot API support

[piksel]

This adds support for sending slack messages using the bot API instead of using a webhook.
It also enables the icon_ props from #173 as they are fully supported by the API.
The documentation would need to be updated with information about the differences between the tokens (and what fields doesn't work work with (non-legacy) webhooks).
This also changes the slack service URL (with backwards compatible parser) to account for the new token types:

Bot API:

xoxb-123456789012-1234567890123-4mt0t4l1YL3g1T5L4cK70k3N
=>
slack://xoxb:123456789012-1234567890123-4mt0t4l1YL3g1T5L4cK70k3N@CHANNEL

Webhook:

https://hooks.slack.com/services/WNA3PBYV6/F20DUQND3RQ/Webc4MAvoacrpPakR8phF0zi
=>
slack://hook:WNA3PBYV6-F20DUQND3RQ-Webc4MAvoacrpPakR8phF0zi@webhook

This was also done in order to easier remove credentials from configuration URLs. The goal for v1.0 is to have all credentials (where practical) in the password part of the URLs.

[codecov]

Codecov Report

Merging #179 (cc5bb51) into main (5d12d75) will increase coverage by 0.41%.
The diff coverage is 84.61%.

@@            Coverage Diff             @@
##             main     #179      +/-   ##
==========================================
+ Coverage   63.65%   64.06%   +0.41%     
==========================================
  Files          86       86              
  Lines        2713     2758      +45     
==========================================
+ Hits         1727     1767      +40     
- Misses        836      839       +3     
- Partials      150      152       +2     

Impacted Files	Coverage Δ
pkg/services/slack/slack.go	75.55% <75.67%> (+10.33%)	⬆️
pkg/services/slack/slack_token.go	82.05% <81.57%> (-6.42%)	⬇️
pkg/services/slack/slack_config.go	100.00% <100.00%> (+7.40%)	⬆️
pkg/services/slack/slack_json.go	100.00% <100.00%> (ø)
pkg/util/generator/generator_common.go	80.26% <100.00%> (+0.26%)	⬆️

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