feat(slack): add thread/reply support

[parrasajad]

• Added thread_ts field to Config and JSON
• If thread time stamp is supplied it will be sent as reply to message in a thread
• Reference https://api.slack.com/messaging/webhooks#threads

[piksel]

LGTM!

I will add this to #179 as well, perhaps also adding logging of the TS of created messages to make it easier to use?

[codecov]

Codecov Report

Merging #182 (47206cb) into main (64eda4a) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #182      +/-   ##
==========================================
+ Coverage   63.57%   63.58%   +0.01%     
==========================================
  Files          84       84              
  Lines        2693     2694       +1     
==========================================
+ Hits         1712     1713       +1     
  Misses        832      832              
  Partials      149      149              

Impacted Files	Coverage Δ
pkg/services/slack/slack_config.go	92.59% <ø> (ø)
pkg/services/slack/slack_json.go	100.00% <100.00%> (ø)

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