feat: add support for custom root CAs

[piksel]

Custom CA support added to the common webclient (pkg/common/webclient), which is now used by slack, telegram, pushbullet and gotify.

Additionally, the SMTP service got a custom implementation.

Adapting the rest of the services to use webclient should be really easy and can be made separately (although releasing with this only supported by some services would need a lot of documentation updates).

Needs integration testing (and more unit tests) and a Router API.

[codecov]

Codecov Report

Merging #185 (41c0918) into main (57b43a3) will increase coverage by 12.16%.
The diff coverage is 67.92%.

@@             Coverage Diff             @@
##             main     #185       +/-   ##
===========================================
+ Coverage   64.06%   76.23%   +12.16%     
===========================================
  Files          86       90        +4     
  Lines        2758     2748       -10     
===========================================
+ Hits         1767     2095      +328     
+ Misses        839      470      -369     
- Partials      152      183       +31     

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	91.66% <ø> (+25.00%)	⬆️
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/gotify/gotify.go	72.34% <54.54%> (-4.59%)	⬇️
pkg/services/telegram/telegram.go	71.42% <75.00%> (-1.30%)	⬇️
pkg/common/webclient/client.go	77.14% <77.14%> (ø)
pkg/common/webclient/service.go	82.14% <82.14%> (ø)
... and 37 more

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