[CIS-526] Expose thread participants as a set of ChartUsers instead of a set of UserIds

[dmigach]

Update the way we expose thread participants. Rather than UserId we should expose them as @CoreDataLazy set of _ChatUser.
Also a part of this task was to make sure we serialise and save participants correctly, this seems to be fine. The only thing I noticed is that backend doesn't include imageURLs for participants into MessagePayload

[codecov]

Codecov Report

Merging #998 (4055b58) into main (f9d8246) will increase coverage by 0.16%.
The diff coverage is 73.33%.

❗ Current head 4055b58 differs from pull request most recent head 18c9a2e. Consider uploading reports for the commit 18c9a2e to get more accurate results

@@            Coverage Diff             @@
##             main     #998      +/-   ##
==========================================
+ Coverage   89.48%   89.64%   +0.16%     
==========================================
  Files         210      210              
  Lines        8758     8771      +13     
==========================================
+ Hits         7837     7863      +26     
+ Misses        921      908      -13     

Flag	Coverage Δ
llc-tests	89.47% <73.33%> (-0.01%)	⬇️
llc-tests-ios12	86.14% <73.33%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
Sources/StreamChat/Database/DTOs/MessageDTO.swift	94.39% <42.85%> (-1.05%)	⬇️
Sources/StreamChat/Models/Message.swift	90.00% <100.00%> (+1.62%)	⬆️
...ources/StreamChat/Database/DatabaseContainer.swift	96.89% <0.00%> (+0.77%)	⬆️
.../Utils/InternetConnection/InternetConnection.swift	54.95% <0.00%> (+0.90%)	⬆️
...urces/StreamChat/Database/DTOs/AttachmentDTO.swift	68.44% <0.00%> (+0.97%)	⬆️
...ocketClient/Engine/StarscreamWebSocketEngine.swift	30.95% <0.00%> (+30.95%)	⬆️

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[VojtaStavik]

I'm not sure about having the lastThreadParticipant convenience variable on the model directly. Maybe move to the file where it's needed in the UI SDK? Even the isPinned there was probably a mistake :/

Once fixed I think we're good to merge this 👍 Also, please add CHANGELOG entry for this change ;-)