feature: ground work for rich message support

[piksel]

This contains some cherry-picked commits from #108 that should make it possible to continue working on rich message support.

Using this, the services can begin implementing the support.

This will not pass Codacy and Codecov checks due to it mostly just add unused shared helpers and interfaces.

[codecov]

Codecov Report

Merging #121 (d2a0913) into main (c7b9fb9) will increase coverage by 1.80%.
The diff coverage is 81.03%.

@@            Coverage Diff             @@
##             main     #121      +/-   ##
==========================================
+ Coverage   58.54%   60.34%   +1.80%     
==========================================
  Files          55       55              
  Lines        1165     1223      +58     
==========================================
+ Hits          682      738      +56     
  Misses        434      434              
- Partials       49       51       +2     

Impacted Files	Coverage Δ
pkg/router/router.go	60.86% <0.00%> (+0.86%)	⬆️
pkg/util/partition_message.go	95.45% <95.45%> (ø)
pkg/util/util.go	100.00% <100.00%> (ø)
pkg/util/util_kinds.go	100.00% <100.00%> (ø)
pkg/services/xmpp/xmpp.go	27.27% <0.00%> (-8.73%)	⬇️
pkg/services/rocketchat/rocketchat.go	14.28% <0.00%> (-3.90%)	⬇️
pkg/services/smtp/smtp_encmethod.go	62.50% <0.00%> (ø)
pkg/services/standard/standard_failures.go	0.00% <0.00%> (ø)
pkg/services/smtp/smtp.go	82.01% <0.00%> (+0.58%)	⬆️
... and 2 more

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