feat: add discourse service

[piksel]

Builds on WebClient from #185. Also needs tests etc.
Supports creating topics, banners, private messages and posts (replies to topics).
Returns errors from the API to the consumer app:

go run ./cli send discourse://discobot:****@disco.ketsueki.meta/banner -t 'Shoutrrr Test' -m 'Shoutrrr test message'
Error: discourse API: "Title has already been used"
exit status 69

Closes #216.

[codecov]

Codecov Report

Merging #217 (d46f560) into main (e46ecc1) will decrease coverage by 0.21%.
The diff coverage is 71.05%.

@@            Coverage Diff             @@
##             main     #217      +/-   ##
==========================================
- Coverage   76.95%   76.73%   -0.22%     
==========================================
  Files          85       93       +8     
  Lines        2703     2828     +125     
==========================================
+ Hits         2080     2170      +90     
- Misses        441      471      +30     
- Partials      182      187       +5     

Impacted Files	Coverage Δ
pkg/services/gotify/gotify_json.go	0.00% <0.00%> (ø)
pkg/services/slack/slack_config.go	100.00% <ø> (ø)
pkg/services/telegram/telegram_json.go	94.44% <ø> (+5.55%)	⬆️
pkg/util/util.go	72.72% <ø> (ø)
pkg/util/util_tls.go	0.00% <0.00%> (ø)
pkg/services/smtp/smtp.go	79.05% <46.66%> (-2.97%)	⬇️
pkg/services/discourse/discourse_post_type.go	50.00% <50.00%> (ø)
pkg/services/gotify/gotify.go	72.34% <54.54%> (-4.59%)	⬇️
pkg/services/discourse/discourse.go	74.41% <74.41%> (ø)
pkg/services/telegram/telegram.go	71.42% <75.00%> (-1.30%)	⬇️
... and 14 more

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[dorianim]

@piksel what is needed to get this merged?

[piksel]

It needs #185 to be merged and tests.